Maybe it is just me - I was underwhelmed with Fantasmic. Had REALLY been looking forward to doing it - but never managed to fit it in with our WDW trips and *finally* got to do it with a dining package. So, we had decent seats, but still had to get there very early - and...surprise! - meh.
I found the show long and kinda disjointed, and the bleacher crowds VERY close - there are frequent "keep moving together, people, smoosh towards the center of the arena, leave no empty spaces.." announcements and it is packed. The effects are cool, but giving up such a big chunk of time...
Good luck with whatever you decide!