First off, there is no "other" provider, and quite likely there never will be.
Ok, that said, I have had both and prefer Sirius. I am also a stockholder in both companies, so it is fine with me, no matter which way you choose!
It is ALL about the prgramming, with the exception being that Sirius does not yet have a fully 100% portable unit that you can listen to like XM does, meaning you can listen to it while you are walking around, keep it in your pocket kinda unit.
XM has MLB, Opie and Anthony, and NASCAR ( though NASCAR is switching to Sirius next year, I believe ).
Sirius will Stern in January. They also have the NFL, and the FULL March Madness basketball tournament.
XM has a "slightly" better choice of music played on individual stations. Sirius uses "focus groups", and eneds up with a smaller playlist than XM. Both services have "SIMILAR" stations. Both have lots of news channels. Sirius, hads local weather and traffice for most major cities. I do not know if XM has that, but I would guess that they do.
I prefer Sirius, but either are good.
EDIT: because I saw the post above this. I get better reception with my Sirius, so, obviously, YMMV. 