I have Sirius (haven't activated it yet because you need money to do that
), but I used my uncle's XM on my honeymoon road trip.
I used several 3-day internet free trials (they go by email address, so multiple email addresses means multiple 3-day free trials) on Sirius, so if extra content (NFL, MLB, NASCAR, etc) doesn't matter, I strongly suggest using free trials to gauge which music channels you like better.
XM was pretty good. I liked the MLB content, which came in handy when in the car for long stretches (heard a few O's and Nats game on my trip), and I was also able to listen to Tony Kornheiser (he's on channel 150 from 11a-1p). However, I settled on Sirius because I preferred the music channels over XM's, and will be able to get NFL and NASCAR (2007, they are on XM til then).
as they are the same price now, it's all in which you prefer, whether it be MLB, NASCAR, Opie and Anthony, ACC, Pac 10, Big 10, MTV, VH1, or NFL, NBA, NHL, Jimmy Buffett (Radio Margaritaville starting next week), and Stern.