I've had both. Go with SIRIUS. They have less channels that are edited for content, more "edgy" selections, and are expected to over take XM in suscriber numbers soon. If you like rock, Octane is awesome. Alt Nation is pretty good, Faction has some of the hip-hop/rock crossover stuff and punk rock, you've got Hair Nation, Jam_On for DMB type music, Buzzsaw has some edgy classic rock. All my brothers (who are in the 35-45 range) have switched or originally went with SIRIUS. They love it.
Oh yeah, what usetosellhummer said, XM played a nasty trick on us former suscribers (which is why I switched). They added all these "new" channels, and then they said they'd be adding commericals to some of the old music channels. So basically, they wedged in one new channel for every old one that was overtaken by commericals.
Their variety of music just sucks anyway. About the only cool XM-specific channel is Cinemagic, but you can live without it.