I think you're hearing the differences but your conclusion is wrong.
I've had Sirius since 2004 and XM since around 2006. Various radios with both. I've heard Sirius sound great and XM sound muddy and then XM coming back and sounding great and Sirius sounding tinny, or whatever over the years. I remember early on, before they really started trying to cram in as many channels as possible, when sound quality for both was actually quite good.
There are a few factors involved:
- compression (digital): I don't have any numbers to back this up but, just from my observations, I'd say that both sides are roughly the same now. It certainly doesn't sound as good as it used to but it does sound OK
- tuners: I think that this is probably where the largest difference lies in that some tuners, regardless of the platform, sound better than others.
I have both Sirius (Directed SCC1) and XM (SiriusXM Onyx Plus) in my car currently, all feeding through the same amp and speakers and I'd say that they largely sound the same. Probably the biggest difference is that there appears to be more audio (not digital) compression coming out of the Onyx Plus. It does make the Onyx Plus sound better (punchier) but it's more a trick of the tuner than anything.
I think that most of the factors in good sound quality, at this point, really come from: tuner, stereo, and environment and it really isn't as easy as saying that one sounds better than the other.. It's more like saying, "This particular tuner using this particular platform sounds good/bad in this particular environment."