We like them both!!! We usually go to the Sci-Fi for lunch and the 50s Prime Time for dinner.
We love the atmosphere of the Sci-Fi, and it makes a nice break, especially on a hot day. I don't think the food is wonderful, but they do a perfectly fine job with the basic hamburger and fries, etc. And I DO think they make a delicious chocolate milk shake!! We really enjoy the old movie trailers as well.
The Prime Time seems more like a dinner place to us, because it reminds me of the home cooking I had as a child. We go for dinner and get things like fried chicken, so it's a heavier meal. The setting is always fun - we usually go a little early so we can sit in the lounge and watch the TVs there. We have had all sorts of different servers - some very "into" their roles and others just good wait staff. It's always fun, though, because even if your server is quiet you'll be able to see into another room. Last month my DD13 and I were there for dinner, and the man at the next table had to put his nose in the corner because he wouldn't keep his elbows off the table!! Fun!
If I had to choose and it was lunch, I'd say Sci Fi. If it's for dinner, Prime Time. But you won't go wrong with either of them!!!