Sirius is 12.95 a month but they have cool options like lifetime subscription for ~400 and yearly as well. They also give you all 120 channels straight up, no add on packages like XM. You can also listen to any stream and channel on your computer at no extra charge (basically like a free in home tuner.)
I think XM is 9.99/mo, Sirius had a LOT more dance/electronic channels with live broadcasts, as well as some awesome talk shows (Motor Trend and Sirius Speedway.) No brainer to me.. here are the pics.