I enjoy Sirius... we have Sirius in the install bay and XM on the sales floor...
In my mind, if you like to hear popular songs more, then XM is the way to go. They pick songs that everyone likes and plays the crap outta them. I swear if I hear that Panic at the Disco song one more god damn time, I'm gonna maim someone.
On the other hand, Sirius tends to have a wider variety of music that they don't repeat as often every day. However, the downside of Sirius is that it seems like they have a 24-hour playlist that they repeat every day, just scrambled. You only hear each song once a day, but you hear it every day.
So in my mind, Sirius is the lesser of two evils because I'd rather have more variety in one day then hear the same songs every day over and over.