I deal with XM at work on the new GMs all day, and I hate it. I cant stand the DJs, the song choices, I think, are never right, and they dont have any of the "extras" that I liked such as stern, NFL, nascar etc.
Sirius I had for year and I f'n loved it. I thought it was the best thing on the earth. There are much more "styles" of music if your not into mainstream, and the DJ, in my opinion, are much better. also, sirius had the playboy channel
One thing I did notice though. The talk stations on sirius, no the music ones, did have some commercials (like stern) to pay for it. The music stations had quick Sirius satelite commercials but that was it. Also, with sirius you get internet radio, not sure about XM.
finally, I have noticed this on both of them, but it seems to be genre based. Some stations dont, some do. Both of them on certain stations, seem to play the same songs over and over again sometimes, and it gets a little agrivating. Almost have to switch from country to classic rock for a few days etc. just to not hear everything 100 times.