PrimeTime 50's would be my choice.
I haven't been here, but I have been to Sci-Fi. The food was ok, wasn't bad, but it wasn't really exciting.
It was cute to first walk into the dining area and sit in the car, but the cute wore off by the time we got our food.
For someone who hasn't been, it is fun to go, but the fun doesn't last. I found that the "trailers" showing were like watching commercials. It sort of ruined the "atmosphere" of the drive-in.
It would be better if they were short little 10-15min movies. I understand the need to keep things short, as showing movies, people would stay and watch them. But the trailers are to short....
I have heard wonderful things about PrimeTime's food, so I would choose that.