I hate when people respond to one of these "which-one-between-these-two-choices-would-you-pick" questions when they've only eaten at one of the places....yet here I am doing it!!
Have not been to Brown Derby, but we make a point to hit Mama Melrose each trip. We really love the atmosphere. As I've said here before, it's not a "Disney-soaked" atmosphere inside. It's a nice, comfortable "normal-looking" restaurant...a nice little escape from Mickey and Minnie, while still being in the middle of the park. So you eat an enjoyable meal and then step back into the world of WDW!
We really love it.