I'm going to vote Sci-Fi all the way. The theming is terrific, the clips are funny (and what kid doesn't love giant monsters?), and it's a fun little taste of an experience that's rapidly vanishing in real life: being with the family at the drive-in.
50's is way, way too hit-or-miss. The idea is that your server is your mom/dad/whatever, and your dining party are sitcom characters. If you're into this, and your server is a good actress, it might be a fun experience. If you just want to eat (which is -and this is important- usually the feeling most little kids have at a restaurant), and/or your server isn't into it either, you are in for a very long mealtime.