They are both fun places for kids.
*Sci-Fi - Very unique & fun dining experience. Everyone should eat there at least once. This place is ALL about the atmosphere in my opinion. The food is fine (not great). All in all, it's worth it.
*50's Prime Time Cafe - As far as food goes, this place has the better food (in my opinion). We eat there on every visit. Love the fried chicken dinner (very good).
Sci-Fi has better atmosphere and 50's Prime Time has better food. If you put experience above food, I would choose The Sci-Fi, otherwise I would go to 50's Prime Time.