Since day one, it has been the mantra about these two....
If you like a LOT of variety, including songs not heard of for years on terrestrial radio, then go for XM. Their catalogs are much deeper (they even run a regular "History of Popular Music", where they play just about every single ever released, in order, from the Depression Era to today... As a former college DJ, I LOVE this. They also have Cinemagic, a channel for movie music, and U Pop, a channel for top 40 music around the world.
However, if you feel radio should be more like a comfortable blanket, playing mostly songs you are familiar with, then stick with Sirius. It's a personal choice.
Me? I now have both in my car. I like baseball AND football. I have a SkyFi2 hardwired into my car, and I recently got a Sirius One receiver that is portable and can be transported from car to car. It looks like a radar detector with a long antenna wire and a long power cord. Fits perfectly in what used to be the ashtray in older model years of my Aurora, but is now just a coin well... I myself find listening to the XM more enjoyable when listening to music, but I do listen to a lot of the talk channels and football broadcasts. Not a big Howard fan though. I used to listen to O & A a lot, but I have kind of drifted away recently....