I've had Sirius for a long time and just added XM also. I like Sirius for the Howard Stern show. Sirius doesn't have the same amount of ground repeater stations as XM does, so if you are in an area with lots of high rise buildings or a city not aligned well to the Sats, you'll get better signal reception from XM.
I also think XM's audio compression codec is distinctly superior to Sirius. Both sound great compared to regular FM, but XM sounds the best.
Sirius has more talk radio if you like that. I like the rock stations on Sirius better. I like the Jazz stations on XM better. The pop and news channels are pretty much equal.
Do what I did, just get both.
