Been in single stories on 4th and 5th floor above Screen door, and last two-story above Thimbles and Threads. Have not been in 3037 or the other third floor GV but I think both have somewhat wider balconies, but all the balconies are fine. Views are good from all the GVs (I consider the views the best at WDW) but it is actually 3037 that might have a minor issue. It is low enough to be even with the top of Hoop Toss on the boardwalk and thus may have a little view blockage when looking straight out. Note that fireworks at Epcot are in the distance above trees, can be easily seen from all GVs and you see only the higher level fireworks. Note that if you look to the left from a GV balcony over YC you can also see far in the distance some high level MK fireworks. (Note, as to any fireworks, I am assuming they will be happening when you go, and under the covid restrictions my understanding is that has not been happening yet).
Though the 2-stories are nice rooms, my preference is definitely the single story. The key differences that favor the single-story other than not needing to use stairs: (a) the great room (combined living room, dining room, and kitchen) in the single story is larger than in the two-story; (b) the single-story has a large, conveniently located, laundry room near the master bedroom; the 2-story has no laundry room; instead just a stacked washer/dryer on the second floor in a small closet at the end of the hall right next to the bedroom that you usually put kids in, and if you use the w/d at night, they are noisy and keep anyone in that room awake.
