I'll refer to an Interview of Gunther Rall from HistoryNet's WWII Magazine September 1996 edition as more of an opinion piece for this first part.

1. With Regards to the Spitfire

At the beginning of the war we flew short-range missions and encountered Spitfires, which were superior. And do not forget the Hurricanes. I think that the Supermarine Spitfire was the most dangerous to us early on. I flew the Spitfire myself, and it was a very, very good aircraft. It was maneuverable and with good climbing potential.

Certainly the Spitfire was excellent, but it didn’t have the endurance of the P-51

2. With Regards to the P-51

I had a very good impression of the P-51 Mustang, where the big difference was the engine. When we received these aircraft we flew about 300 hours in them. You see, we did not know anything about how they flew, their characteristics or anything before that. In the P-51 there was no oil leak, and that was just fantastic. This was one of the things that impressed me, but I was also very interested in the electrical starting switches, which we did not have. This made it very difficult in starting our engines in the Russian winter. We had the inertia starter. The cockpits of all of these enemy aircraft were much more comfortable. You could not fly the Bf-109 for seven hours; the cockpit was too tight, too narrow. The P-51 (cockpit) was for me a great room, just fantastic.

I don't have much aside from that interview with regards to German impressions of the P-51 and the Spitfire. That said, I'll add that Rall's opinion as one of the higher scoring aces probably gives him a bit of weight with regards to his evaluation on flying the planes.

Now compared to actual combat performance, the strengths of each plane are hard to compare given the massive difference and massive differential of air superiority between 1941 and 1944-1945. But if anything, I hope this small bit I have can enlighten you a little bit of German opinions of either aircraft.

Source:

WWII Magazine, September 1996

The Germans certainly did evaluate a number of Allied aircraft, both bombers and fighters. Many captured aircraft ended up at the Rechlin test center where they were either repaired to flight status or dissected to discern technical secrets or for use as spare parts to keep other Allied aircraft airworthy. In general, the Luftwaffe pilots' opinions of Anglo-American fighters was actually quite good, but these evaluations were colored by their own experience with German aircraft.

When on leave in the summer of 1940, Werner Mölders had an opportunity to evaluate both a captured Hurricane I and early Spitfire at Rechlin. His remarks in a letter back to JG 53 on both aircraft demonstrate how he evaluated Allied aircraft by the standards of his own experience with the Bf-109:

The Hurricane is a bit of a tugboat with a retractable undercarriage. In our terms both are very easy to fly, the Hurricane in particularly good-natured, steady as a rock in the turn, but well below the Bf-109 when it comes to performance- it’s heavy on the rudder and the ailerons are sluggish. Take-off and landing of both types is child’s play. The Spitfire is one class better, very nice to the touch, light, excellent in the turn and almost equal to the Bf-109 in performance, but a rotten dogfighter as any sudden dive and the engine cuts out for seconds at a time and because the propeller is only two-pitch (take-off and cruise) it means any vertical dogfight at constantly changing heights it’s either continually over-revving or never develops at full power at all.

The interesting thing about Mölders's evaluation is that the metric he uses to evaluate both types' virtues and faults is the Bf-109. For example, his letter makes positive reference to the RAF aircraft's ease of take-off and landing (child's play). The Bf-109 was a notoriously tricky aircraft on landings and takeoff; poor brakes, narrow undercarriage, and torque from the DB600 series engines meant that its pilot really had to keep his head about him or would crash the plane. The 109's controls were also quite heavy, so the Spitfire's light controls were something that Mölders would notice. Mölders condemnation of the Spitfire's dive characteristics also reflected his own experience with the 109. The early Merlins had a float-type carburetor that would cut out during a dive, something that would not happen to the fuel-injected Bf-109. The fact that the British aircraft engines would stall during what was a routine maneuver for German aircraft likely influenced this negative assessment. Later variants of the Merlin would rectify this problem by 1941 and the RAF was already on its way to fitting constant-speed propellers on its aircraft when Mölders made his test flight at Rechlin.

As the war dragged on, Rechlin acquired a growing stable of Allied types for evaluation. By 1943, a special Erprobungskommando-style unit had been formed at Rechlin under the command of Theodor Rosarius. Nicknamed the Zirkus Rosarius (Rosarius Circus) both tested Allied aircraft and traveled to various airbases to demonstrate Allied aircraft to German pilots. As the aircraft were unique, only flight leaders were allowed to fly the Allied aircraft in these visits as they would be less likely to make a fatal mistake. Walter Wolfrum, an ace with JG 52, recalled his experience with both a P-47 and P-51:

During the war I had the opportunity to fly captured P-47s and P-51s. I didn't like the Thunderbolt. It was too big. The cockpit was immense and unfamiliar. After so may hours in the snug confines of the 109, everything felt out of reach and too far away from the pilot. Although the P-51 was a fine airplane to fly, because of its reactions and capabilities, it too was disconcerting. With all those levers, controls and switches in the cockpit. I'm surprised [their] pilots could find the time to fight. We had nothing like this in the 109. Everything was simple and very close to the pilot. You fitted into the cockpit like a hand in a glove. Our instrumentation was complete, but simple: throttle, mixture control and propeller pitch. How pilots were able to work on all their gadgets and still function amazes me.

Despite these reservations, Wolfrum rated the Mustang as one of his most dangerous opponents during the war. One of Rechlin's test pilots, Hans-Werner Lerche had a similiar reaction to Wolfrum when he had to fly a repaired P-47 from Caen to Rechlin in November 1943:

The P-47 cockpit could baffle even an experienced pilot. I gradually managed to ascertain the functions of most of the levers and instruments, but there were some obscure controls, the meaning of which was not immediately clear to me. For example, the flaps were hydraulically operated [Bf-109 flaps were mechanically operated] and there was one lever which was apparently connected with the compensation of the flap angle on both sides of the wing, but how this should function was at first a mystery to me.

In contrast to Wolfram, Lerche appreciated the roominess of the P-47's cockpit and the cockpit hood, which was all the more noticeable since he "was used to cowering in the Bf-109 seat to avoid continually knocking my head against the cockpit hood, compared to which sitting in the Thunderbolt was flying in comfort indeed." After several incidents in the ferry flight flying an unfamiliar airplane, Lerche was able to put the P-47 through the paces at Rechlin and discovered its excellent diving performance and good handling at high altitude, but noted it lacked decent performance at lower altitudes.

Later marks of the Spitfire and the P-51 generally got high marks from the test pilots at Rechlin. One of the more interesting experiments the Luftwaffe did with these aircraft was to mate a DB 605A-1 to a Spitfire Vb. When examining the Spitfire, the Luftwaffe team found that the engine firewall was very similiar in cross-section to the Bf-110 G, making such a mating possible. The modified Spitfire was found to have better performance with the DB than the similarly-powered Bf-109 G. Lerche found the P-51's Packard-Merlin to be an excellent engine packed around a decent airframe. he later recalled:

After only two flights it had become quite clear to me the Mustang was a truly unique aircraft. Its performance characteristics were very good and the engine too made a very good impression. But also one felt at once that it required careful handling: when revving up it reacted very sensitively to the correct operating temperatures and if handled roughly, it countered with unsteady running. I had the impression that you could hear immediately if the engine disliked something. Of course the Packard Merlin did not like jerky opening of the throttle either, but if the air temperatures were right it ran smoothly and reliably. Very soon the news got around what a special treat we had received with this Mustang and, when examined in more detail in the workshops, it gained general appreciation and was constantly besieged by representatives from all kinds of departments.

In Lerche's comparative flights with the Mustang in mid-1944, he recalled that only the latest models of the Luftwaffe's fighters, the Bf-109 G-10 and FW-190 D-9 were able to keep pace with the American plane. Unlike the Bf-109, Lerche found that when lightly loaded, the Mustang was a fun aircraft to fly that did not require constant vigilance.

So overall, the Germans appreciated Anglo-American aircraft as technically sophisticated aircraft that were not to be treated lightly. In contrast, Luftwaffe reports on captured Soviet aircraft often emphasized the relative crudeness of the cockpit and the simplicity of Soviet designs. Some Luftwaffe pilots were rather taken aback by the sophistication and pilot workload of these fighters, but the closer these fighters' performance approximated German aircraft, the higher the pilots evaluated the mounts of their enemy.

Sources

Lerche, Hans-Werner. Luftwaffe Test Pilot: Flying Captured Allied Aircraft of World War 2. London: Jane's, 1980.

Smith, J. Richard, Eddie J. Creek, and Peter Petrick. On Special Missions: The Luftwaffe's Research and Experimental Squadrons 1923-1945. Hersham: Classic, 2003.

Weal, John. Bf 109D/E Aces 1939-41. Oxford: Osprey Publishing Ltd, 2012.

During the Battle of Britain, Herman Goring asked his squadron leaders what they needed to win the battle. Adolf Galland, one of the top Luftwaffe pilots of the battle (and of the entire war), responded that he would like his own squadron of Spitfires.

So Galland obviously had a high opinion of the aircraft.