I have Sirius and a buddy of mine has XM and they basically play the same music on similar channels. XM is $10 a month for their basic subscription and then you pay extra for their special channels. Sirius is $13 a month and you get all their channels. Also, I think that XM has the rights to MLB where as Sirius has the rights to NFL (You need to check on that if its important to you cause im not 100% positive). I used to have XM several years ago but i decided to get rid of it, no particular reason, I just didnt listen to it very much. After I canceled my subscription they kept charging my credit card for 8 monthes, i called them every month to ask them why they were still charging me and they always gave me the run-around. I finally had to tell my credit card company to stop paying them (guess i should have done that in the first place). Anyway, I never did receive my money back from XM, i eventually gave up because I was tired of the hassle of dealing with them. Every time i would call them they would tell me that they had nothing about me in their system and i got tired of filing out the same form over and over.
That was my main reason for picking Sirius when i decided to get sat radio again. Im sure my case was just some fluke but there was no way i was going to deal with XM again.
My vote goes to Sirius