I also prefer the food at 50's PTC, but I'd vote Sci-Fi: it is a unique setting that you just cannot get outside of Disney World. You feel like you are part of a show - the theming is great. (50's theming is great too, but you can just walk in the Tune In Lounge there and take a peak to see it.
Note that a party of 6 will be split between 2 'cars.' IIRC, you sit 2 in a row, 2 rows per 'car,' so it isn't as if you can really talk to more than 1 person easily anyway (you are all facing the same direction: the film screen). The cars are close together so you won't have a problem communicating across cars.
Enjoy!