What to think about:
This is where the more cutting edge stuff is going on. This is new technology (well, a new application of a tried and true technology) so the choices are limited to two: Sirius and XM Radio.
At this point in the game, there are three major differences:
Programming - The best way to flesh this one out is to give a listen to both providers, looking at their program schedules side by side. Both services also allow you to listen live online. This is the only way to make the choice. If you are looking for specific programming, make sure you spend some time doing this.
Hardware - Both systems have all necessary hardware for home systems as well as car and dual purpose systems. Now that Sirius has released its boombox, both platforms seem to be equal.
I have heard that XMRadio has a better ground repeater network, but I have traveled all over the east coast with my Sirius receiver and have had no reception problems.
Price - XMRadio checks in at $9.95 a month to Sirius’s $12.95. Since you are paying for content that you can otherwise get for free from the airwaves, I would urge you to refrain from considering a three-dollar difference when making your decision.
What I use:
I did some major homework. There's too much info to go into here, but if you would like to see my in-depth review, go here.
Sirius Satellite Radio
This is the one I ended up going with.
The more popular choice.
A great online source for satellite radio gear.
They carry both providers, and you can hear the gear live. This is where I bought mine.