Sci-Fi is by far the most popular restaurant in Hollywood Studios, no other place comes close. There aren't alot of sit downs is HS.
What I would recommend is keeping your lunch time if you can. Lunch is the best time to eat at Sci-Fi and a bad idea during the night. You want to be as flexable as you can during the night. People will be lining up for Fantasmic early.
I would go to Sci-Fi for lunch to escape from the sun after riding the Great Movie Ride and then;
at night eat at ABC Commissary which has great food, seating and is relatively quite because everyone is trying either to get on HTT or RR, or waiting for Fantasmic.