If you'd like to get XM, the only option thus far (that I know of) for seamless integration using the steering wheel controls is XM Direct. However, to my knowledge, it is incompatible with Nav & DSP. Had it in my '04 X3, and LOVED it! As for Sirius, I have tried it as well, and the only real difference is in the sports category. Sirius, for the most part, has a moratorium on professional sports, except for baseball. As for the various genres of music, there really isn't much of a difference IMO. XM Direct, of course, is less expensive than the Sirius BMW receiver. There's not much of a difference in the monthly cost.
For more info on XM Direct:
http://www.xm411.com/phpbb/viewtopic.php?t=1804