I haven't had any experience with Sirius, but when I had XM, I didn't really like it that much. Needless to say, after my free trial, I didn't renew the subscription.
I'm more of a music fan than I am a talk radio fan. When I would listen to music, it would be no better than listening to local radio. XM stations still played to most popular songs over and over, just like local radio stations do. I thought having it would mean more variety in music, more underground music, old school music, not the same five or so songs played every 20 minutes.
I have heard that Sirius is better for music, and XM is better for talk radio.
Also, towards the end of my trial, which was towards the end of the summer, XM started having commercials. I thought the whole point to paying for radio service is to enjoy commercial free music. I suppose cable tv started out commercial-less also.
I can't comment on Sirius from personal experience, just from what I've heard.