I own both XM and Sirius. XM portable unit (Delphi MyFi) for O&A, which I have had for a couple of years. Sirius because I bought a new Jetta a year ago and got 3 months free, then decided to keep it in my car. I have also connected XM in my Jetta.
I commute 60 miles a day to and from work and by far, XM has the better technology as it RARELY disconnects or shows "LINKING"; however, the music stations content is not as good as Sirius. Sirius, on the other hand, has better music content and stations, but the technology sucks, as it is always showing "LINKING" and cuts out.
I called Sirius to find out if there was either anything I could buy or something I could do (since it was a factory-installed car radio), and (1) they do not have a solution to the problem and (2) was told by one of their tech support people that the reason it cuts out is because "their sattelites are spinning in orbit and XM's are not". As a result, they noticed that there was an "error" in setting up my service, and gave me 5 months service for free. So I still have both XM and Sirius in my car, but it drives me crazy every time the signal goes out with Sirius because it does not happen with XM in the same areas.
I would call them up and tell them you have both XM and Sirius, and XM doesn't give out in the same places and maybe they will find an "error" with the way your account was set up as well.