It's not because I think that XM's programming is better in fact I feel it's slightly worse. I'm not a sports fanatic so that didn't play into it.
About 12 (?) yrs ago I bought and installed a Sirius Starmate Replay unit and enjoyed it in three different cars. About a year ago the power connection broke and I decided to buy the latest Starmate5. I noticed right away that the FM transmission wasn't anywhere near as strong as the previous unit was. No matter, my new Sonata had an Aux plug so I connected it that way. Then I began to notice that the sound quality was sub-par to the XM signal which at one point I was receiving through my Sonata's radio thanks to the 3 month free trial. It sounded like it was under water or like if you were playing a record and walked by the turntable with a heavy foot, called flutter I think. Anyhow I eventually contacted them and they tried resetting my unit but to no avail. I put up with it for awhile longer until I was surveyed on my Sirius customer service experience. They contacted me afterwards and eventually they sent me a reconditioned Starmate5 to try. Now I'm in a new 2013 Sonata and the free XM trial that came with that car sounded 100% better than the plugged-in reconditioned Starmate 5. So today I cancelled my Sirius and towards the end of the 3 month period I'll sign up for XM.
I'm going to miss the Starmate though. I really liked the replay function as well as the ability to program favorite artists or songs and the way it beeped every time one was playing on any one of the stations. I also liked being able to listen to NPR which apparently requires the more costly XM premium package. That'll be a subject for discussion for sure.
So it would seem that the sound problems were related to the Starmate head unit vs. compression issues since the XM sounds very CD-like. On the plus side Sirius customer service as above average in my opinion.