I've had Satellite radio for a while now, but I'm an XM guy, not a Sirius guy. But looking at the two radios you linked to, the Sportster seems to be a more deluxe radio. It's got a full color display as opposed to the Stratus' single color display. The Sportster's got a fancy rotary knob, and a Replay feature that lets you pause, rewind, and replay live radio which the Stratus doesn't have. The Sportster also looks like it's got a feature that let's you tag favorite sports teams so that it will track scores and stuff like that for you.
You also may be able to find cheaper refurbished models of these at
www.myradiostore.com
Personally, I go with the Pioneer Inno, it's pretty much an MP3 player that you can carry around with you and get live radio wherever you go, and you can get cheap docks or accessories that let you use it with your home speakers and your car speakers. It's nice to have if you walk a lot like I do or want to listen at work or whatever. Of course, the Inno is an XM radio, not Sirius. But i'm sure Sirius has similar portable radios.