I've stayed at all four, and they are all very similar. There are differences, but they are not a significant factor in deciding where to stay. A few things I noticed (YMMV):
If you do stay at an All0Star, check which one has been recently renovated. We were in a 'new' room as ASMu, and it does make it a lot nicer.
Pop's food court was the most hectic.
The All-Stars share buses during 'off' times. If you go early, return late, you'll have your own bus.
That said, AS-Music is the best (by a small margin) of the four for bus service. You get picked up second, and ASMu sometimes fill up shared busses. ASMovies is out of luck if that happens. On your return trip, you can take whichever of the three All-Star lines looks best to you, because ASMu is a really short walk from he other two resorts. If you don't need to go to your lobby/food court, your room is likely only 1-2 extra minutes from the other resorts.
Pop bus lines may be long, but they do have more buses running most of the time I've seen. A couple times, we walked out to see a HUGE line for MK, but the buses rolled in fast, and they frequently had us loading into a second bus while the first one opened up for the wheeled folk.
We stayed at Pop last year, and really loved it, and are doing a bounce-back to Pop this year. But I would be happy to stay at any of them.