Hi Karon,
The SCI FI has always been a place that we NEEDED PS ressies. It's one of the few places that we have never been able to get in early if we wanted to eat earlier than our stated time. There is always a line and we always have to wait a little even with PS. We like to eat lunch there, and we try to make the first PS of the day at both Sci Fi and Prime Time. I have always seen a line of people waiting at Prime Time too, but since it's close to the front of the park we usually only eat here at 11am when the restaurant opens. We only have to wait a few minutes.
The Sci Fi is one of our family favorites. Not so much for the food as we usually find it hit or miss on being fabulous, but the atmosphere is so much fun. The kids really enjoy dining in the cars, the old sci fi movie clips are a scream and its nice and it's just a fun atmosphere.
Last year they had a new sandwich that had steak and gorgonzola cheese with onion rings. That sandwich was out of this world! Probably the best meal I ever had there. DH had it as well and he thought it was fabulous as well. The hamburgers have always been tasty there as well and the onion rings are WONDERFUL!
Prime Time is also a great restaurant with a fun theme and atmosphere. The food is also excellent and I think the food has always been more consistant here. The Salmon has always been a favorite, and there was a wonderful steak sandwich here as well that was served open faced with a wonderful dressing! The milkshakes here are great (love the PB&J shakes).
I have read plenty of bad reviews on both restaurants, and I guess its like anything. It depends on the day, who's cooking and what your tastes are.