I have eaten at prime time 3 times and sci fi twice. I definitley suggest Prime Time. why?
we went with our boys to each. they got a kick out of the teasing at prime time (although I did have to go "encourage' the aunt to come to our table and "bug" the boys) they were ages 9 and 12.
at ages 11 and 15, they liked sitting in the cars at sci fi, but, to be honest, kids don't remember Drive-ins (like we do) and don't recall all those old sci fi movies, so the "point" was kinda lost on them.
since then (the boys are now grown and gone.. why do we say that? gone? like I don't know where they are?

although sometimes.. I wonder

) hubby and I have eaten 2 times at prime time and tried once again at sci fi. the FOOD is hands down better at prime time. the chicken is great!!!also, the pot roast! the onion rings are to die for! I love the desserts!
the ribs at sci fi were... eh. the burgers... eh. check out the restaurant reviews and I think you'll see the food at sci fi is not stellar.
when we went with older boys (who were not at all into "characters" )we wanted meals with a little something different, and they did have fun at both. I would suggest both, but MGM is really only a one-day park..
my vote.. Prime time