Hmmm . . .
I'v installed XM Direct in my E46 (Sirius-ready headunit, HK Premium sound) and am quite impressed with the audio quality -- not to mention the content.
We went with the factory installed Sirius in my wife's E90 (about a month out from delivery as of today) because (a) we had to have satellite, (b) we wanted to compare Sirius to XM (which we have in my car, her outgoing car, and in the house), (c) there is not an XM Direct adapter out for the E90 (yet), and (d) because I' was stoked about the NFL broadcasts.
I'm now a little concerned that you guys are not happy with the audio quality of Sirius. I had heard that the signal coverage is not as good as XM (more dropouts), but never that the audio was substantially worse.