Hey, congrats on your trip later this month (I'll be in Disneyland Resort) and I hope you have a blast. I've eaten at both, so here's a quick summary...
50's Prime Time Cafe: You will get many 50's "enthusiasts" and I am not one of them. That atmosphere of the place is really a fun, casual one where you are picked on by sibling relatives who make you eat all your food, keep your elbows off the table, and vice versa. This 50's Prime time is essentially an ATTRACTION as I consider the food to be below par. Last time I went there (FEB 2005) we went there for a lunch and the food was okay, nothing special. And a big part of the 50's Prime time Cafe is the waiter/ess. If the waiter is not particually involved in the storyline, this dining spot may turn out into a total bust for both fun and food.
Brown Derby: I love the food here. It's simply spectacular and the atmosphere is so relaxed and yet it seems miles away from a themepark. The food is excellent, the experience is wonderful, and you should expect a much more "adult" meal at the Brown Debry. I highly recommend it- they have excellent vegatarian entrees and desserts. Try to fit in dessert here- I really can't think where this meal can go wrong!
Whatever you eat at...enjoy!