Two scenarios:
1) You see the 9:00 Fantasmic! In order to get the best seats, you should probably show up 60-90 min early (more towards the 90 min side though). After the show, you can keep your seat and watch the fireworks from there. And while the audio does play from over there, you don't get the same experience as you would if you watched the fireworks from right in front of the castle. It is a pretty good view of the stuff up in the sky though.
2) Watch the fireworks from in front of the castle; however I would recommend definitely getting there 90 min ahead of time to ensure a good spot. Seeing the fireworks from that area is ideal and truly a wonderful experience, although you do have to stand for the entire show (as opposed to option #1 which you sit and watch the fireworks from the Rivers of America). Then when it is over, scramble through the mass crowds and try to get over to the Fantasmic seating area, while you have hordes of people who were doing option #1 now going in the opposite direction as you trying to exit the park. Then you can try and find a good seat for the 10:30 Fantasmic show, which requires less waiting time than the first Fantasmic show, but that is because there's only like a 1/2 hour between when the fireworks ends and when the 2nd Fantasmic starts.
I've, personally, done both. It really depends on which sounds better. If you're really tired, go with option 1. Option 2 is good because you can get decent spots at both shows. The major drawbacks are that you have to watch the later Fantasmic show, which doesn't end until 11 and, overall, you spend more time waiting.