I've never eaten at 50's in 15 (can't believe it either) visits to WDW. With that said, if you have to pick one, from what I've heard, 50's is better. If you have the opportunity to do both, definitely try Sci-Fi. If you don't care about eating in a car, you can get a seat quickly as a walk-in at a table. Though the menu is limited, the drive-in atmosphere is really fun. The CM's are doing the movie trailers with the movies. It is a fun place to eat and the milkshakes they serve are really good.