This might be the post you are talking about. As I've said before, the sound of the 50i is probably the #1 reason to get it over the 35i. From this post, it sounds like a decent reason to get it over the M as well
http://www.xbimmers.com/forums/showthread.php?t=745737
PS - I went with the 50i over the M for the following reasons
a) Color choices
b) Active Stering
c) residual/cost (it leased out significantly higher because of a much lower residual).
Still the X6M does turn heads