Actually, Sirius has the better satellite transmission. Sirius beams their signal at a much steeper angle so interference is less likely. XM has so many repeaters because they NEED them.
Also, I have both XM and Sirius tuners with my D3. The sound quality of Sirius is so much better than XM. I have the latest XM tuner and the previous SIR-PNR1 Sirius tuner. The XM tuner is faster but the music is very flat.