I have an XM Commander installed in my Jetta and love it.
I've had it about 18 months...when I decided to spalsh into the satellite radio world, Sirius' viability was still in question, so I went with XM. Nowadays, Sirius looks like it's going strong, so either one should be around for awhile.
One other factor in picking XM is their radios looked better in my car. Everything Sirius offered at the time was ass-ugly and about as big as my Sony Walkman from 1988.
Content wise, the music channels are pretty much the same, although I've heard Sirius has more content in that regard.
XM has:
-NCAA football/basketball for Big 10, ACC and Pac 10
-MLB (every team)
-Opie and Anthony
-Traffic and weather for many larger cities
Sirius has:
-NCAA broadcasts for select teams
-NFL (every team)
-NBA
-NHL (if the league doesn't go bankrupt, haha!)
-Premier League Soccer
-Howard Stern
Both have Traffic & Weather for most major cities and the ability to listen online at no extra charge.