My personal preference is Mama Melrose. We love the menu, and we have a favorite CM who almost always is our waitress named Barbara. She makes our meal so special! We love catching up with her. I think for a lot of people it might boil down to the menus. Have you checked allears.net to compare the menus of the two restaurants? Mama's appeals more to us, plus we have found that the "antics" of the wait staff at 50's can be very hit or miss, so if you are thinking of going for the "show", you might be disappointed. That being said, we have been before and had lots of fun.
Good luck with your planning!