When I was trying them both back about 2 years ago I thought they wee nearly the same. Over time with my Sirius and a buddy that has XM I hear only a slight change leaning toward Sirius. I voted too close.
I find (now) XM to sound "less full", less depth or range. I have listened to both on a few separate auto and home systems; namely my own where I am acutely aware of the quality, power, sensitivity and responsiveness; from the amp to the loudspeakers.
I really can't tell much of a difference but I do remember reading somewhere that Sirius' music is a little less compressed than XM's even though they are compressed ALOT. So because of that, Sirius got my vote.
I run both through the same head unit and powered sound system. This last Spring XM lost a lot of sound quality to me and is not as good as Sirius. Of course this is my opinion and both are far superior to FM radio.