When I bought a new 2004 Chevrolet Tahoe, it came with XM standard along with a three month free trial subscription. I thought, "what a dumb idea, who would ever PAY to listen to the radio?" Well, after a few weeks I was hooked and now I am an enthusiastic customer. Now, I hardly ever listen to my CDs or to conventional radio in the car. I recently bought a Delphi "MyFi" which is their handheld portable XM receiver. It is about the size of an iPod (a little bigger) and works great at home (it comes with a docking station that hooks up to your stereo), on the go, or in our other cars. It has a built in FM transmitter that to my ears, sounds great played through the car radio. And the MyFi has a couple great features I really like. First, you can record and save up to five hours of programming, favorite songs, etc, and can set it up to record favorite programs you know are coming on (like a VCR). Second, it has a feature called "TuneSelect" where it will beep to alert you when your favorite songs or artists are coming onto any channel. Pretty cool. Very well done and easy to use device. Best of all, as an existing XM customer they offered the MyFi for $99 including free shipping, activation, and some cool bundled accessories.
If you are not a customer, I think places like Best Buy, Circuit City, etc are good because they can sell you the radio and get you preactivated, and they are still eligible for the $50 rebate (Walmart and Target as well as XM direct are not eligible, but their prices are not $50 less).
XM vs. Sirius, can't help you there. I love XM but have never tried Sirius although I know they have fans too. The XM music lineup has everthing I could want and I really like the local weather and traffic channels. I am a hockey and baseball fan (XM has both) but not as big an NFL fan (Sirius has it). Sirius carries NPR which some people find important.
Good luck and cheers,
Dave