Sirius has a bigger bandwidth and can broadcast video; they just don't yet.
XM just launched a second satellite; Sirius already has three up. XM currently charges for listening online; Sirius lets you listen online free. XM might charge you extra to listen to every channel they offer. Sirius is more expensive.
Either way, satellite radio is just awsome, regardless of which one you choose.
I went with an integrated built-in kit, so it's car only (but I didn't like the idea of having a huge external unit hanging around somewhere in my car sticking out and getting in the way). I wouldn't listen to it in the house since I have a 125-disc CD changer and internet access at home.
To everyone else: If you don't have satellite radio, go get it!!