I love them both. Sci Fi is the one and only restaurant we have been to in all seven of our WDW trips. Prime Time is a hoot and almost like a dinner show if you get a server that is into their role!
Sci Fi: Good burgers, shakes and reubens, extremely quiet and relaxing, the whole place is watching the screen. Its a wonderful place to have a break, eat and recharge. I prefer lunch, if you do go at dinner and want a burger, ask, they aren't on the menu but they are happy to let you order one.
Prime Time: Lots of comfort food selections, its a kick to see the old kitchen decor and if you enjoy interracting with your server all the better. If you don't I wouldn't recommend this one.
If you plan on spending two days at MGM, I suggest both! They are both themed, fun dining experiences, hard to experience anywhere but WDW and since MGM counter service options aren't the best IMHO, its nice to have two fun sit downs to choose from.