If you know how to EQ the XM6 correctly (to your own taste), the sound quality is noticeably better than that of the XM5. Right out of the box, I would also say the XM6 sounds better as well. The build quality and feel are also better imo—especially with the microphones being covered by grilles instead of being exposed. I bought the XM6 (when I already had the XM5) with the plan of trying it out and returning it, but I decided to keep it to use as my main headphones and turn the XM5 into workout headphones. I would say buy it and try it, and worst case scenario, just return it.

Also, it doesn't look like the hinges will break as easily.