I've not been to both so I don't know why I'm chipping in but!!.....
I go to the Sci-Fi every trip, we love the atmosphere and enjoy the movie clips, I hate dining out but here it's entertaining. It's a bit dark but I like that, I can shove lots of food in my mouth without being seen!
Personally I wouldn't do the 50s Prime Time, I'm a bit shy and would not appreciate all the antics, I would love to try the food though. But this is one place I would never go.