I have both (BTW, XM and Sirius are the only 2, not just the major 2

well, atleast in the states...) and like most things, they both have their pro's and con's.
XM's news and traffic is better, in my opinion for the simple fact that they don't combine 2 area's onto one channel. For example, I live in Pittsburgh and on XM channel 215, I get Pittsburgh area weather and traffic. Where as with Sirius, on channel 155 I get Pittsburgh... and Detroit... It's kind of nifty, as my dad lives in Detroit, but otherwise I find it to be a pain in the rear.
Because we're on a Disney board, I should mention they both offer Radio Disney.
Other big differences are sports programming. XM has Nascar and MLB, where Sirius has NFL, NBA and NHL. Of course if you want Stern, say hello to Sirius.
I personally like Sirius's music programming better, especially for country (though keep in mind, I listen to everything from metal to classical to pop to bluegrass...). In my opinion, XM sticks to a specific genre to much, IE XM's "metal" channel Liquid Metal was very hardcore HEAVY metal. While I like metal, this is a bit to hardcore for my liking. Of course, that reason may be why Liquid Metal no longer exsists on XM. Sirius seems to mix it up a bit more in their stations, bringing in artist's that might borderline a specific genre.
XM does have a few channels that I wish Sirius would duplicate, one of which being Cinemagic. The channel does nothing buy play movie soundtracks, which is very cool (atleast I think so

) I've heard everything from Finding Nemo and Monsters, Inc. to Aliens and Star Wars.
This bothers some people and some it doesn't, but Sirius doesn't have "live" DJ's. For the most part, everything is pre-recorded and additionally, you can never speak to a DJ. If you want to say something to them, you leave them a voicemail. Occasionally they'll play that voicemail, but cleverly edit it and "script" what the DJ says to make it sound live.
Lastly, hardware wise, XM's is IMO a bit nicer. The Roady2 and SkyFi2 are both topnotch XM units. XM DEFINATELY has Sirius beat in the size arena. XM's units are packed with as many features* but are considerably smaller and especially thinner. *Sirius does have one huge benefit, they now have their Replay units out. Basically, think Tivo for your radio. You can pause, rewind and fast forward your station. GREAT for talk-shows when you get out to pump gas, stop at the drive thru, etc etc.
As I said at the beginning, they both have their pro's and con's. I would highly recommend checking out a retailer that carries both so that you can compare side by side. I started with XM and picked up Sirius about a year later. At this point, I listen to Sirius ~65% of the time and XM 35% of the time. I doubt i'll ever get rid of either one of them.
HTH
