For the best quality, I would recommend getting a serious ready kenwood deck and get the sirius tuner with it. With this combo you can control sirius from the radio instead of having another tuner mounted on your dash. Also, the sound quality is the best because it is a direct connection between the tuner and radio, instead of the other fm modulated versions.
Someone posted on Brad's site that because Delphi is so tied in with GM & XM radio that they are playing catch-up with Sirius. They said that it probably won't be until 2007 that there is an option on the Mustang for a sound system with built in Sirius.