You basically have to see what the programming is on each and base your decision on that. I have Sirius and love it. Of course, I'm a Howard Stern fan. If you like humorous talk radio (check out the Blue Collar Comedy channel. You'll bust a gut!), most pro sports, and completely commercial free music of every variety, I highly recommend Sirius. They were late starters, but they're kicking XM's ass these days and will probably surpass XM in subscribers before year's end.
Sirius also has no premium channels. Once you sign up, you get everything. Not so on XM. They offer some of their programming at a premium. Sirius also has some excellent pre-paid deals. Pay for a year and you get a free month, two years and you get two free months, etc.
Good luck! Either way, it beats the hell out of terrestrial radio.

and no, I don't work for Sirius . . . .