I have Sirius and I love it. I can't speak to the signal quality of XM because I don't have it. I get a great signal with Sirius. It might depend on the part of the country that you live in. I'm not sure. You might want to check on that.
Since I'm an old coot, I'm sure that you would be interested in different stations than I am. I mostly listen to Classic Vinyl and The Vault (two classic rock stations) I also listen to a Garage Band station (I forget the name) it has some great alternative stuff (they also have a station called Alt Nation). But I also listen to my old foggy stuff - a station called Classic Radio that plays old radio shows from the 30's and 40's like The Lone Ranger, The Shadow, The Green Hornet, Dragnet, etc. ANd the Elvis Channel - Elvis, 24 hours a day. LOL
Sirius also plays all of the SEC football games on Saturdays and the NFL on Sundays. (home AND away radio crews).
You can check out all of the stations that are available at
http://www.sirius.com