Some other options to consider:
1) You will need to pay a little extra per month for each radio
2) You can download a program from
http://www.siriusmac.com (if you have a mac) which will allow you to listen to sirius through your mac (at no other cost). You could go thought sirius.com, but this program automates the login process and gives you a cool stiletto looking interface.
3) I fogot what three was for
4) If you have an iPhone, you can get sirius ANYWHERE, if you can get a cell signal (through the built in Safari web browser), which is more likely than using their portables. For instance, you can get a good signal inside a building which you probably will not be able to get with their portables.
5) You can pay a nominal cost for a higher quality feed.
I originally got Sirius for Stern...but have grown to like a lot of the other channels...Raw Dog Comedy (104) for one. I only listen to FM now for quick local traffic (WTOP).