Depends on the ages of your kids.
It's really what goes easiest on them.
I'm not much of a buffet eater, but many people hail Hollywood & Vine because it gets the food in your body and you out the door fast. Good for fussy kids.
Sci Fi is really entertaining, and especially well themed, but service in peak hours (12-2) have pretty slow service, so it may backfire. Of MGM eateries however, this is my can't miss. Many people hate the food, truly despise it, but I like it. Then again, all I ever get are onion rings and a burger. I think if you stick with the "traditional" you'll be fine.
Mama's is tasty, and fairly quiet - In my opinion, I prefer this as an adult. The kid cousins last year weren't really all about it.
50's is well themed, but I've actually never been, so I can't give an opinion. It seems to always be refurbing when I go (as it is again, this week, as I'm leaving on Thursday...*sigh*)