Jamie,
I guess it depends on how much you are running your AR and how you charge it. Right now, I'm doing a lot of teaching with my AR and, upon examining the factory handle, I've noticed increased wear on one side of it. I cahrge the rifle using my support hand and by grasping mostly the latch and left side of the handle only.
Plus, the BCM are more robust than many of the factory offerings.
As to large vs. med. I'm looking for the medium but I'll take what I can get. I've read that these handles have a tendency to hang up on gear when the rifle is hanging or slung due to the size of the latch.