I failed this choice for our upcoming trip and ended up booking both.
I prefer the food at 50s, and the antics can be really fun. The last time we were there, my then-15yo tried tattling on his then-5yo sister for not eating her veggies, hoping to get something started in her direction, and our server punished him (along with a gentleman from the next table who got caught using his smartphone at the table) by having him stand up and sing "I'm a Little Teapot." We were DYING!
But at Sci-Fi the restaurant itself is amazing, like nowhere else we've ever been. The food isn't great but we've never had a bad meal there either - it is just Applebees quality burgers and wings kind of stuff - and the trailers on the big screen are just plain cool. We have learned over the years to stick to the simpler menu choices; this isn't the spot where I'd order a steak or pasta dish, and inevitably the times we've strayed from the bar food items on the menu we've been a bit disappointed in our choice.
If I absolutely had to pick just one, I'd probably give 50s the edge but I really don't think you can go wrong either way, as long as you know what you're going for.