I chose XM (Score 1)

I got XM in December of last year before driving cross country. At that time, Sirius wasn't up yet. Reception was perfect all the way from Houston to LA. Reception only fades when line of sight to the satellite is blocked (tunnel, tall building, hills, etc.) and there are no local repeaters.

One big difference for me is that XM has CNET Radio and Sirius does not. That said, Sirius has some channels that I wish XM had (NPR, PRI). Also, it's great to have 24/7 Bluegrass.