Both are very good restaurants. I would say the food at 50's is a little better, but you just can't beat the sci-fi theme. The 50's CMs can get a little "annoying" at times, at least for me.
I don't really like the same old routine they do when your trying to eat. But, the last time I ate there, I think they could sense I really was'nt in to it and we had a peaceful meal.
If I had to chose one I would pick Sci-Fi. There must be somthing about watching old movies and commercials that gets me.