Despite the considerable overlap in programming, a handful of distinctions are so clear that you can base your decision entirely on them. Baseball fan: XM. Football nut: Sirius. Movie maven: XM. Howard Stern addict: Sirius. Bob Dylan freak: XM. NPR lover: Sirius.
There's more in the article, of course -- it's well worth a read.
One of the reasons I haven't subscribed to either service is the short shrift classical music receives from both services. Granted, the goal is to build subscribers, but something other than the Basics 101 would be nice.
So what would I like in a classical channel? I'll save the detailed rant for another time, but in brief, I'd like a focus on living composers, complete works (no excerpted movements), unusual repertoire, a wider range of composers and style periods, and a better representation of chamber and vocal music. And part of me thinks that the general classical audience would like some of that also – if they only knew there was a choice.