The X5 middle muffler is a near true dual design, its straight through so little restriction with some perforated pipe at either end. The X pipe tends to be more of a bottle neck which tends to give that raspy rushed gas sound, I've never been a fan personally. They also make the overall tone quieter.
The best change from the middle muffler would have true dual pipes with a small 1" H pipe, that would have kept the X5s V8 rumble but made it louder. Still, each to their own. I'm just obsessed with exhausts :p