If you like jam bands (Phish, the Dead, etc.) and you like a lot of jazz, as I do, then there is no contest. Sirius has a jam band station, XM simply does not. The jazz stations on Sirius are better as well, and there are more of them. The rock stations on Sirius are really good, too, and I don't know how they are on XM as I am not an XM subscriber. If you listen to top 40 or alternative or something, then it probably won't make much difference, as both companies have these stations. I have Sirius in my car (the Direct Connect one that plugs into and is controlled by the head unit, as opposed to the plug and play ones that use an internal FM transmitter) and I listen to it pretty often. I also have an iPod 60GB, and a head unit with an auxiliary input. This was vital to me, as all the FM transmitters for mp3 players sounded absolutely horrible to me (and I tried about 5-6 different brands).
I've been a Sirius subscriber for a couple of years now and I've been happy with the programming. There are really only a few stations that I listen to regularly, and to me the good music, as well as the absence of commercials, is well worth the monthly fee. All I had to do was look at the list of stations for each company, and the decision was pretty easy for me.
I don't know if XM also offers the free trial, but with Sirius you can sign up for a free 3-day trial that lets you listen online. Maybe that would help you choose.
