I've had both. I started with XM, then switched last year to Sirius just for the NFL, then went back to XM when the season ended. Both have good and bad, and here's my opinion on them:
XM - the good: I really like some of their stations - the 70s, the Loft, XM Cafe, the comedy channels. They play full-length versions of the songs. Some funny bits and sound bites (from movies, the Simpsons, Seinfeld, etc.) are played between songs. Cinemagic station is cool. They have 3 comedy channels, one of which gets Phil Hendrie. And the selection of units is better.
XM - the bad: they tend to play the same stuff over and over, especially on the comedy channels. No NFL.
Sirius - the good: More diverse play list. NFL. Seems to have a bit more "variety." Howard Stern coming soon.
Sirius - the bad: They play the chopped, FM versions of songs - a big annoyance. A lot of their stations run together, meaning that you will hear the same songs on five different channels. Less of a selection of units.
And...this is purely subjective and my own opinion, but it seems to me that XM is geared more towards the 30s and 40s crowd and Sirius is more for the 20s. Being almost 40, I prefer XM.
Just my .02