Welcome TedS.
I have both, here are some differences:
Sports:
NFL - Sirius
MLB - XM
NBA - Sirius
NHL - Both, but XM only starting in 2007
NASCAR - XM, but Sirius starting in 2007
Music:
XM has a much deeper playlist, including bands & songs you haven't heard of before.
Sirius plays songs you have heard of before and has a shorter rotation.
Shock Jocks:
XM - Opie & Anthony, Ron & Fez, Monsters in the Morning, and others supposed to be announced soon.
Sirius - Howard Stern, Bubba the Love Sponge, and others to be announced soon.
Hardware:
XM is 1 generation ahead of Sirius in their chips so XM's radios are smaller.
If you haven't yet, check out
http://www.xmfan.com/ for XM and
http://www.siriusbackstage.com/ for Sirius. You will find a listing of receivers and features as well as a lot of good info.
Bottom line is you will be happy with whatever service you choose.