Each has different programming, so go with the one that suits you best.
Besides each of them having 60-70 commercial free music stations there are some differences when it comes to the other programs they offer;
Sirius - NFL, NHL, Howard Stern (in Jan 2006)
XM - MLB, Nascar, Opie & Anthony
Both seem to have college sports as well, but i believe XM may have the edge in that dept.
As for sound quality, thats a toss up because everyone will have a different setup, but they are very comparable. XM uses a set amount of bandwidth per channel so the talk and music channels have the same general quality (although the amount of compression varies), while Sirius can vary the bandwith per channel, giving higher fidelity to the music stations and less to the talk (which dont need it). What's the result? Ehh, not much IMO, its hard to really notice a profound difference.
One last thing to keep in mind is price. Sirius is $12.99/mo and XM is $9.99/mo, but it gets confusing. With the $12.99 on Sirius, you get all the channels they offer, web streaming of all the music channels on your computer, and 1-3 months free if you prepay a year. On XM, $9.99 gets you most of the channels, but if you want O&A or Playboy radio you have to pay an additional monthly fee (O&A - $1.99, Playboy - $2.99). Also with XM, if you want web streaming its an additional $3.99/mo.
Whew.... Personally i went with Sirius because they seem to have more offers from time to time than XM does, and their programming is excellent. Just last week i picked up the Sirius XACT plug and play unit for free when i prepaid a year of service (w/ 1 month free).
Check out these sites for more info;
http://www.xmfan.com
http://www.siriusbackstage.com