For pure food, I probably wouldn't go to either, Mama Melrose and Brown Derby are better.
So that leaves the experience.
As others have mentioned, to really enjoy 50's prime time, you need a great server. Also, you have to like the experience and interacting with someone you don't know and pretend they are your cousin or family member. For Sci Fi Diner, it's whether you think you would enjoy sitting in a "car" watching shorts on a huge screen.
If I had to choose, it would be Sci-Fi diner, partly because I haven't done that in a while,, and we more recently did prime time, and partly because I'm more of an introvert and thus don't really get the most out prime time.