I have Sirius and I love it. I've never listened to XM extensively, so I can't say for myself, but I have heard that XM playlists are much deeper; too deep for some (meaning they play too much stuff you don't know). For me, Sirius has a good mixture. The hits channels (like Alt Nation in the Rock category, for example) play well known stuff while other channels (like Left of Center for rock) play lesser known stuff. It's all about personal preference, so I'd try out the online trials at both websites.
One thing XM has that I wish Sirius had (and have written them a few times about) is the Cinemagic channel, which plays motion picture scores. It's great! But Sirius has
2 NPR channels, has all 3 hours of the Dave Ramsey Show (which for some reason I'm hooked on), and of course Stern is coming.
I have the Audiovox PNP3, which I like a lot. Here's a decent deal for the receiver and a car kit:
http://www.ecost.com/ecost/shop/detail.asp?dpno=519278
I use the boombox at work:
http://www.ecost.com/ecost/shop/detail.asp?dpno=517531
Also, check out
www.siriusbackstage.com Great Sirius forum. They have this Product Matrix, for example, that will help you choose a receiver:
http://www.siriusbackstage.com/product_matrix.php