We aren't fans of Sci Fi though its been years since we have been. I would like to give it another try sometime, but can't get the rest of the family to go because they remember this...you couldn't talk to each other because its like sitting in a car, you all face forward, and the food was less than stellar. The film they play is neat and seeing it once, is cool. Seeing it over and over, not so much.
50s on the other hand has been on our must do every trip list since 1999. The food is always good(though we have been on a couple occasions that we were disappointed a bit), the antics can be subdued or way out there. I think a lot of times the servers can pick up on whether you want to play along or be left alone. Its a cool setting, set in the 50s, old tv shows playing every where, but the same, sit there long enough and you will see repeats.
Based on my personal experiences with both, 50s would get my vote every time!