I've got both services. I would say to try out each service on line (free trial) and try and sample as many channels as you can, or at least the musical genres that intrest you.
Again you will run into a very "heated" debate on which service is better, but either beats AM//FM hands down. Check out
Sirius Backstage and
XM Fan more info.
The S50 cannot receive live content yet, but Sirius will have a "live" portable out in 06. We don't yet know if it will be a add on to the current S50 or a new device.
As far as the music goes, I like Classic Rock and "jambands" in these genres Sirius wins in my opinion. XM's playlist is very deep and sometimes too obscure for my tastes.
For sports, if that interests you, it depends on which sports you like. Sirius has the NFL and NBA locked up, while XM has Baseball. For now they share the NHL (XM has a GREAT hockey talk channel!!) but in 2007 it will be an XM exclusive. They also both have plently of college sports.
Sirius has been a bit behind in the receiver department. Some say though, and I agree, that while the Sirius units are larger and "hotter" than the XM ones they seem to have a better designed interface.
I woudn't worry about either service going belly up anytime soon. Both are growing by leaps and bounds. If anything worry about what you're going to be hearing out of your AM/FM radio!
EDIT: I don't know where you get a receiver and pay via paypal. I've used The Sirius Store and can recommend them, but they don't take paypal.