I have xm and a delphi roady 2. I tried the fm modulator and found the quality to be horrible, or atleast not any better than the wireless transmission. I took the thing back. It may be better for you though, since it will be made by a different company, etc. I think the best way to go is to buy the aux adapter for the stock radio, crutchfield has it, someone has posted about it before (I'll try to search)... anyway, I believe it plugs in where you would add a cd changer, and makes you able to hook up the radio through the line out part of your satellite radio. The quality would be the best that way, I would think. I'll try to find the post and/or link to that part from crutchfield and post it on here for you... hope this helps.
Edit: the fm modulator also made my normal FM stations worse.....