I have had both and I like XM for their programming and sound quality. Serius has mostly unknown or unsigned artists (not really the good ones either) and the comedy channel is not half as good as XM.
XM has more comercials but still only mabye 5% of what you would hear on FM radio.
i have been split, i wanted to be able to hear howard stern, but neither one has him, i think the xm system would work better for me, i drive in my van and would like to listen to something decent, i think the only station that reach out in the desert are mexican talk radio. looks like its gonna be xm for me.thanks