Neither really. Part of the problem was that Japan was fighting a naval war it could not win against a foe that could outbuild it. The historian H. P. Willmott outlined this problem in his book The Barrier and the Javelin :

Such was the scale of American industrial power that if during the Pearl Harbor attack the Imperial Navy had been able to sink every major unit of the entire U.S. Navy and then complete its own construction programs without losing a single unit, by mid-1944 it would still not have been able to put to sea a fleet equal to the one the Americans could have assembled in the intervening thirty months.

Some IJN admirals were aware of this problem. Vice Admiral Inoue Shigeyoshi called a war with the US as akin to playing chess without the possibility of gaining a checkmate. Other IJN planners thought that a series of sharp blows delivered to the USN and a strong defensive barrier would lead to the US abandoning the war. This was wishful thinking, but the apparent ascendancy of Germany in Europe, the vulnerability of European colonies in SE Asia, and the threat of American embargoes forcing Japan to abandon its expansionism joined together to create a certain logic for Japan to embark on a war it could not win.

Of the two battles listed- Leyte or Midway- neither were "decisive" in that they were a make or break battles. Had Japan won at Midway, the USN would still have all those Essex-class carriers working up or on the slipyards and all Japan would have gained was a position on an atoll it could not supply or defend over the long-term. The occupation of Attu and Kiska demonstrated the weaknesses of Japan to supply such garrisons that were at extreme lengths from its main bases.

But at least the odds at Midway were evenly matched. Parshall and Tully's landmark book on Midway, Shattered Sword, has noted that the battle was less miraculous than prior historiography and popular memory holds. USN airstrikes had the advantage of attacking carriers that were extremely vulnerable to such damage and all available evidence suggests that the Japanese were not about to launch an attack minutes before the dive-bombers hit smashed three of them. But while Shattered Sword has demystified some aspects of Midway, it was a closer-run thing than Leyte. Here the disparities Willmott outlined were readily apparent. The Japanese had reconstituted some of their air groups after the mauling of the Philippine Sea battles, but Halsey's airstrikes prior to Leyte destroyed this effort. The IJN chief of the 2nd Air Fleet Fukudome Shigeru likened his planes' counterattacks to throwing eggs against a wall. Japanese planes were not only quantitatively inferior to their enemy, but the yawning gap in quality was now apparent. Much the same could be said of Kurita's surface force at Leyte. It was powerful on paper, but really stood no chance against the Americans over the long-term. Had the IJN pushed its advantage against Taffy 3 at Samar, it would not have changed the grand strategic picture.

Japan fought a war it could not win and no single battle could change this fact.

To echo what /u/kieslowskifan has said, even if the US had lost the Battle of Leyte Gulf or Midway it would have been difficult for them to lose the war without simply giving up on their own. The die was cast fairly early on, not just with one dimension but along many.

First you have naval power. The US had vastly superior naval manufacturing capabilities, and exercised them to their fullest extent during the war. They not only built a huge arsenal of carriers, destroyers, airplanes, and so forth but they built an equally if not more impressive logistical tail that stretched across a huge chunk of the Earth. There was an expressway delivering war materiel right onto Japan's doorstep hot from factories across the US.

Second you have the use of anti-shipping warfare for interdiction of trade, supplies, troop transports, etc. Even when the US wasn't firing on all cylinders in this area due to malfunctioning torpedoes they still had vastly better doctrine and organization. While the Japanese on the other hand were using their anti-shipping capabilities (especially submarines) hugely ineffectually in comparison. The Allies sunk over 90% of all Japanese merchant shipping tonnage in 1942 alone, a loss that the Japanese never fully recovered from. Meanwhile, in Liberty Ships alone the US built 6 times the total shipping tonnage that the Japanese had in 1941 and added that to an already substantial supply (not all of which went into the Pacific War, of course, but it shows the scale). While the Japanese sank a comparatively tiny amount of tonnage (1/6th of what US submarines managed) and focused mostly on taking out warships instead of interdiction.

Third you have the training, techniques, and technologies. As the war progressed the US got better and better with air sorties from carriers. They were able to coordinate air to air combat through long range radar and radio, something the Japanese were comparatively inexpert at. Additionally, the US had superior training processes, they rotated the best pilots back home to teach and they had the resources to allow new pilots to have a considerable amount of air time and to undertake live fire exercises. And on top of that the fire fighting and damage control designs, processes, and training for US carriers was superior to the Japanese carriers.

There are several inflection points where these differences hit home, one is the Battle of Midway where 4 carriers were hit by dive bomb attacks throughout the battle, resulting in horrendous damage, uncontrollable fires, and sinking within a matter of hours. On the other hand, the US carrier Yorktown was hit by comparable strikes but damage control systems and procedures were so good that within a matter of hours she was not on fire and was steaming at a steady clip, she appeared so healthy that she was hit again by Japanese bombers who mistook her for one of the other carriers. Even after that it took a torpedo attack from a Japanese submarine two days later for Yorktown to finally go down.

Another inflection point would be the Battle of the Philippine Sea, aka "The Great Mariana Turkey Shoot". By this point the Japanese naval pilots had deteriorated badly, many of the best having been killed in action previously (such as at Midway) and the IJN's training program being unable to replace them at the previous levels of quality. Those pilots faced off against highly trained and well coordinated veterans of air to air combat, flying in many ways superior planes. Even though on paper the numbers were pretty close (roughly 900 US aircraft vs. about 750 Japanese aircraft) the result was that nearly 90% of the Japanese aircraft were destroyed while less than 15% of the US aircraft were.

The Battle of Midway was one of the first instances where it showed that even though the odds on paper might be fairly even between US and Japanese forces the reality was significantly different. The US got lucky in that battle a few times (though they had plenty of bad luck as well), but as they say luck favors the prepared, and definitely disfavors the unprepared on the wrong end of it. Japanese carriers proved to be extremely vulnerable in battle. The result was a hugely disproportionate outcome compared to what one might have expected going into the battle. And that pattern kept playing out repeatedly after that. The Japanese were facing devastating loss after devastating loss (not just in set piece battles but in interdicted shipping, destruction of naval bases, and so on).

Logistics doesn't always win wars, but certainly in the Pacific War it was a key deciding factor. The Japanese never came close to keeping the lid on American logistic capabilities, they were consistently improving year over year. While on the other hand the US was bleeding Japan dry. Cutting off its ability to move war materiel around, cutting of its supply of key resources like gasoline, and even, ultimately, beginning the mass starvation of the home islands.