Well it kind of depends on what you listen to... I have Sirius and I love it, though a buddy of mine has XM and it seems to be pretty good. I've heard some say that Sirius has more depth as far as playlists, they have certainly have great variety with rock stations. I like classic vinyl, classic rewind, hard attack (heavy metal), first wave (new wave), left of center (indie), which are some of my favorites, two pretty good hip hop stations hip hop nation and shade 45, plus an old school hip hop station. Though I don't listen to those channels only, I flip around a lot. Plus they have some electronic, dance stations... all in all I'm sure XM has a similar lineup.
Then of course there is a ton of talk, including Howard Stern, comedy channels, new, sports radio, etc. The one thing I wish I had is MLB, which you can only get on XM. I love the NFL, but the MLB has a much longer season and baseball games are great to listen to on the radio, that's the worst part about Sirius, imho, but that's a minor complaint. I'd trade the NBA, nascar, college, even all the sports talk so I could have MLB and the NFL.
You can't go wrong with either, though I'd recommend Sirius simply because I've been really pleased with their service, I don't know as much about XM.