Both are good. Maybe hours will change some and you can see both.
Fantasmic is great. Ton of characters, good music, not a lot of fireworks, lasts about 30 min
Star Wars is really great. We've seen it two or three times now and are still wow'ed by it. Mainly a projection/sound show, but still has plenty of fireworks. You don't have to be a Star Wars fan to like it, but for Star Wars fans it's not to be missed.
Imo, if I didn't have enough time to see both during a trip and won't be back for awhile..I'd see Fantasmic and sit near the top of the arena and leave the show about 1-2 min before the finale..or get up near the end and walk slowly out as you're still watching it. Then go quickly and try to get a decent spot as close to the Chinese theater as you can to see Star Wars. People have done this and have had success. You won't be super close but if you can get along the main road and have a clear view of the Chinese theater, it can work