Ha, good question: The short answer is quite naturally both. Battles are rarely won or lost because one side made a mistake and the other simply existed - battles are won and lost because one side can capitalize on a mistake while minimizing the fallout from their own.

Someone once asked "why did the Germans lose at Stalingrad if they were 'better trained?'".

/u/elos_ was good enough to frame the question as a series of smaller questions, and I think it'll help to just revist the post in general:

What do you mean they 'were forced to rely [on Axis partners]'? In what ways? What were the Romanians and Hungarians roles in the battle and why were they important?

"The flanks of the 6.Armee were in fact held by Balkan allies, such as the Hungarians and Romanians; and their armies by comparison were undeniably backwards technologically to the Germans. A strong case can even be made to their fighting ability - but that's not what Beevor chose to select as a reason for their impact on the battle."

The reliance on qualitatively inferior elements on both flanks of Stalingrad cannot be discounted, and it was something that STAVKA were quick to evaluate in planning Operations Uranus and Saturn (later Little Saturn). Extensive reconnaissance was done to seize P.O.Ws (what the Soviets called "Tongues") and begin an extensive line of research into the equipment, morale and fighting strengths of these Hungarian and Romanian armies. Two things stuck out rapidly in their extensive preparation: The allied armies lacked significant anti-tank defenses and mobile reserves, and that the Romanian and German units were often at logger-heads. Co-ordination between the Romanians and the Germans in particular was practically nonexistant.

I'll quote a paragraph from this linked answer to surmise the Romanian-German military relationship:

The relationship between Hitler and Marshal Antenoescu for example was gruff but amiable, but in no way, shape, or form often translate to respect between Romanian or German troops. Germans derided the larger part of the Romanian's ability to fight (despite recognizing an excellent tradition of mountain warfare) and how the Officers treated their men. There was without a doubt a shared philosophy in their atavistic fear of Bolshevism however - and that is something that extended to other countries in the Axis sphere of influence, such as Hungary. Again though, the relative accord between dictators did not always extend all the way 'to the ground' so to speak. Again using Romania as an example - there was absolutely an effort to build a camaraderie between at least the soldiers. Animosity between Romanian and German soldiers became so disastrous on the Ost Front that the Germans had to issue general orders for dealing with Romanian troops, and Marshal Antonescu suggested and implemented frequent banquets and inter-unit parties/lunches for neighboring German and Romanian units.

The Soviets were aware of this:

Already aware of the tensions between the German and Romanian armies, [an interrogator] was also interested in strained relations within the Wehrmacht.. (Beevor, page 181)

So not only are the Romanian units bordering the 6.Armee lacking in anti-tank weaponry and mobile reserves, but the ability to rapidly assess the situation and react with neighboring German units is in serious doubt. Then of course we have the simple fact that the mobile forces of the 6.Armee were greatly attrited after the heavy and fruitless fighting in September, another fact that the Russians were aware of:

After a badly wounded corporal revealed that 24th Panzer Division was reduced to sixteen tanks... (Ibid.)

The effects on the mobile units effectiveness to block or blunt Russian penetrations during Uranus are duly noted further on in Stalingrad:

Instead of deploying Heim's Panzer Corps en bloc as planned, the 1st Romanian Panzer division was diverted....this seperated led to further disasters... Having failed to organize a mechanized strike force before the enemy offensive, [Paulus] continued to do nothing. The 16th and 24th Panzer Divisions were left with many of their key units bogged down in street-fighting in Stalingrad....

(Ibid., pg. 245)

How was Paulus caught off guard? Beevor is of the firm opinion that had the 6. Armee foresaw the incoming strike with more clarity, sufficient mobile reserves could be organized to blunt armored exploitation of any gains made by the Soviet assaults. Beevor's opinion here is not unfounded, as the Germans still entertained a noticeable qualitative superiority; even disastrously under-strength German armored units could make a mockery of the stiff Soviet tank tactics, something that would remain a norm until 1943/1944. However these mobile reserves would need to be noticeably concentrated, otherwise they would lack the strength and numbers and firepower to stop penetrations. The Germans were unable to effectively concentrate their remaining armored reserves for reasons we'll explore below.

The formidable build up of Soviet reserve units behind the Volga was impossible as a rule to completely hide, but the actual strength and nature of the build-up was greatly aided by several factors:

• Paulus was undeniably lethargic in his command at this stage, and suffering greatly from tunnel vision. Even as the storm broke on the flanks, German units were still bringing certain sectors of the Volga crossing under intense fire. The idea of being 'nearly there' was one deliberately and dangerously fostered by the Soviets, whom fed in only enough troops to General Chuikov to keep him fighting in Stalingrad. The Germans, psychologically and operationally were pinned in the rubble-heap of the city.

• The rapidly changing conditions in weather prevented the IV. Fliegerkorps from conducting as many sorties as usual, robbing the 6. Armee of tactical air reconnaissance that may have been able to reveal more details about the Soviet build-up. The increasing number of VVS units being fed into Stalingrad to contest German air superiority in an effort to further delude such efforts can also not be discounted.

• A certain feeling of disbelief viz. their allies on the flanks undoubtedly fed in to German complacency. When the aforementioned aggressive probes began on Romanian positions, the nervous Romanian commanders frequently called for mobile reserves and German support. The Germans began to dismiss these calls for help, realizing that the Romanians were fighting small and local actions, nothing that truly threatened their sector. This disbelief instilled certain lethargy, only made worse by a paralysis of liason communications at the time of the actual attack:

A surprise Soviet attack on [a Romanian] headquarters destroyed the German liasion officer's radio set, the only means of communication with [Heim's Panzerkorps], and all contact was lost for a few days...

(Ibid, pg. 245)

• Finally, just as Uranus can be considered an embryonic attempt at returning to Deep Battle, the Russians exerted every muscle to maintain the most strict efforts at deception, this art is called Maskirovka and is best described by Glantz and Nipe, some efforts made at Stalingrad include:

1. Concealing the actual movement and preperations for the movement of troops and supplies into the concentration area...

2. Strictly limiting movement of these forces...during daytime

3. Camouflage of the assembly positions and dispositions of the attack force.

4. Elimination of battle reconnaissance

...

6. No firing of artillery that was designated for the support of the attack...registration was accomplished by having roving batteries or already sited guns provide this information.

---

So, returning to the answer: The disaster at Stalingrad was undoubtedly aggravated by German lethargy at the moment of decision, but to simply say the Germans made grave mistakes would be to completely discount the Soviet's back-breaking efforts to ensure these mistakes were capitalized upon. Literal months of build-up in the strictest secrecy, identification of weaknesses in all spectrums of combat and the ability to keep the most mobile German units pinned, spread out or deluded factor in to the RKKA's decisive victory. The Russians would show such vigorous planning and preparation in all of their most successful operational attacks from Stalingrad to the end of the war. The ability to collect and analyze as much information from the enemy as possible while ensuring as little as possible was fed in return to the enemy gave the Soviets an insight into the German situation that they could exploit readily. Capitalizing on the poor efforts at coalition warfare and the technological inferiority (relatively) of their allies also greatly aided the Soviets efforts to secure a victory. The habit of only re-equipping newly formed or newly committed units also ensured that the forces slated for Uranus were impeccably well armed and equipped for fighting on a snow-strewn steppe. The offset of course was that Chuikov's hardfighting 62nd army had to make due without adequate winter supplies. The gamble in general of giving Stalingrad's defenders only what they needed to cling on is another factor in the Soviet victory; the Germans were straining every nerve to clear the city while the Russians were hanging on by their fingernails, allowing the counterstroke to be developed.

It's not that simple of a dichotomy; a key skill in warfare is recognizing and taking advantage when an enemy makes a mistake. That said, the Germans definitely made a mistake in deploying for battle at Stalingrad; the sheer amount of forces set aside for the first assault congested the roads (preventing a swift capture before it could be manned and fortified) and alerted the Russians that their enemy was devoting considerable strength to attacking a highly defensible position.

Turning that into a resounding strategic victory, however, required careful operational planning, deception, and deployment across theaters on the Red Army's part, as well as ferocious resistance inside the city itself. Keeping the Germans pinned at the center, they concentrated their strongest forces against the weakest part of the Axis's line, broke through, and invested the 6th Army, with plans to seize Rostov and thus cut off Army Groups A and B in the Caucasus and around Stalingrad; this was most of the German Army's southern wing. This didn't happen, since relief efforts for Stalingrad disrupted the schedule, but it shows remarkable skill and confidence after a year of stunning defeat.

I think you have to say both.
Hitler stopped the attack on Moscow and started moving to the Caucus region to take the oil fields. But, he split his forces and sent some to Stalingrad, he could of passed on the city and it wouldn't or really mattered.
He might of taken the oil fields and increased Germany's oil supply and cutting the Soviets. Someone asked a Russian soldier why he thought the Soviets fought so hard. He replied "we were more afraid of Stalin than Hitler."