Both places are running more on atmosphere and theming than on the merits of their food. My husband and I ate at both on our last trip. While we thought the theming at 50's Primetime was pretty amazing, the food was anything but. Onion rings were thick and greasy and my husband's meatloaf was brick hard and appeared to be the end of a loaf. Our server was merely adequate, but the table next to ours had one who was seriously into the act, which only made our server seem more lacking.
We loved Sci-Fi, though. My husband was totally into the themeing and our food was better; although all we had was a burger, onion rings, and shakes. But they were delicious! Of the two, Sci-Fi is the only one we are def. planning on returning to.