I prefer Sci Fi. I love the atmsophere, its a family tradition for us, we've done it each trip but one (had a family member get sick so we missed our ADR) and can't wait to go in March. I love their burgers, shakes, onion rings, reuben and they have pretty good ribs too. I also love their sundaes and spinach dip. Its a nice relaxing meal, its generally quiet in there as everyone, young and old is riveted to the screen.
All that said, I do enjoy 50's too. I guess I grew up eating too much fried chicken in Texas because I find the fried chicken at 50's to be very sub par. But I love their pot pie, meat loaf and pot roast and to me the antics are fun since my family enjoys that. But if I was with anyone who was uncomfortable with that, it would be a nightmare!
I think they are both good, just hard to compare since they are each so unique.