Of course if you're not going to have the dealer install the XM tuner, there's no reason to stick to XM. It's just as easy to use Sirius, since you'll be hanging on a separate antenna for whichever you chose.
I went with a model similar to the Roadie for the Sirius service, but chose the other because there's a few different programming options that one has over the other. (I listen to NPR a lot, and Sirius has the exclusive contract for that, plus Howard Stern and Martha Stewart (I'll pass on those two.) Meanwhile, XM has NASCAR and Playboy
The both work the same (I've had both, in different cars) and the billing is basically the same. Plus, if you get radios for multiple receivers (you pay a subscription for each one) then the additional receivers are at a discounted rate. I have two, so it's $12.95 + $6.95/mo for the two.
I don't konw about the XM billing but Sirius is like Tivo - you get a discounted "lifetime" subscription rate of $499 (per receiver) and that can be transferred up to 5 times to new receivers, so as you upgrade your audio equipment (or trade cars, if you have the radio built in) you don't lose the investment. (Meanwhile, a TiVo "lifetime" subs. is good for one receiver only.)
Also (as of today - this stuff changes a lot) Sirius includes free web access with your account, so you can log in on their website and listen to the same programming online when you're at your computer. It's a decent internet radio but neither satellie network is 100% commercial-free, like if you were using a service like MusicMatch Radio or Live365.com BUT... it's "free" with the Subscription (on Sirius - I think XM still chages a small up-sell for web access.)
AND (last one, I promise) both services offer a "home kit" which is basically an antenna that works in your home, so you can pull the gadget out of your car, and use it in your house by routing the sound through your home stereo. That looks like about a $40 add-on for either service (will vary depending on which receiver you bought for your car.)
It's a (much!) easier install in the non-hybrid because of the fold-down seats, so you can stick the antenna on the center/back of the roof (sticks on with a magnet - very car wash-safe) then run the wire into the main compartment and up to the receiver, tuck it in around seats and carpet.
Since you can't get through the back seat panel into the trunk, on the 03, I stuck the antenna on the passenger side roof over the center pillar, got everything hooked up, and then tucked all the additional/loose wiring into the pocket on the back of the seat.
For the 03, it fits right in the little cubby next to the e-brake and is out of the way. The 06 will be a neater fit with that little cubby under the Climate controls. (the one I bought has its own remote, so there's no readhing around behind the transmission lever to switch channels.)