I would make a contingency plan to go on both days. I've had several trips in which the fireworks were canceled due to high upper level winds even though it wasn't necessarily windy in the park itself. Go on Saturday so if it's canceled you may get another chance to see it Sunday. The worst trip I ever had, it was canceled every night except for Saturday -- the one day that we made plans to be elsewhere. I kept telling my DH the fireworks were probably going to go off, and he said no way it's been canceled all week. And sure enough, it went on that night. The next night, Sunday, it was canceled again 5 minutes into the show due to winds. We left the next day. So go the first chance you get, and then you can go again if it's canceled.
ETA: Best place to view fireworks is subjective, but I like parking myself in front of the castle about an hour and a half or so (depending on crowds) before fireworks. You can't beat seeing the fireworks around the castle and seeing Tinkerbelle fly. If you want to wait until later, anywhere on Main Street will have a great view of the fireworks and you can still see Tink - just further away.