The food at 50's PTC is infinitely better than the food at the Sci-Fi. There is really no comparisson.
The atmosphere at Sci Fi is cool. If you go and you want a car, make sure you make your reservations in advance. The seating is a bit awkward and conversation can be difficult. Especially with the movie clips playing on a loop.
50's isn't as visually striking as Sci Fi. But the atmosphere is still wonderful. If you get a good waiter, it makes all the difference. We've never had anything but fun at 50's. We go every single time. And did I mention the food is infinitely better than at the Sci Fi?
Usually, if we do Sci-Fi, it is for dessert. We can soak in the atmosphere for cheap and the desserts are good. (It's hard to mess up dessert.) I wouldn't actually recommend eating a meal there. But if I were going to do so, a late lunch would be best.