XM. Gotta have my Opie and Anthony! Actually had XM since it came out in the other cars and have the SkyFi. They have the best technology and units by far. All the Sirius units look like big fishfinders. And the new MiFi is awesome.
If you're buying a FX-internal SAT, Sirius is your only real choice- there's some serious operational issues with the XM recievers (they'll drop off-line 'till an engine restart if they lose signal). Not only that, they will only display the first 10 characters of a song or artist, and there's no way to see more of it.