I used to sell both, had both of them in my car for about a year each, and this was my conclusion. If you are:

1. Over 40, buy XM

2. 40 or under, buy Sirius

The reasoning is the demographic of people that inquired for both based on music and programming tastes.

Most 40+ wanted alot of Talk, Classic Radio, Blues, Jazz, etc which I believe XM covers better.

The under 40 crowd wanted Rock, Rap, Dance, etc which I felt Sirius had more of a selection.

Its less likely the 40+ crowd is going for Stern, which I will go back for as well.

It also seemed the XM seemed to play the same 50 or so songs on each channel I listened to, and that got boring real quick...

This is just MY opinion, and programming has constantly changed since they both came out, so I say go to a store and listen to each for an hour or so or find a buddy with one, and then make your decision based on that..

I also have a pretty good ear for quality, and the difference in sound quality is minimal. (I still dont believe into the fact that Sirius' quality is worse) Simply put, programming should be the only basis for your decision..