Neither, they both suck these days, and I have both.
XM = Clear Channel
Sirius = Removing channels so they can put single artist channels on.
Bastards.
Let me explain what Sirius has been doing. They had a channel called Planet Jazz #70, it was my favorite channel. What do they go and do? Remove it so they can put the Grateful Dead on 24/7 with their own channel. They took away half their jazz lineup by doing it and the only place to hear new jazz.
They took away channel #11 so they could put on a 24/7 Bruce Springstein channel. Sounds wonderful, but I can't listen to one artist 24/7. Their programming directors don't know what they're doing, which is why the "buzz" with both companies is wearing off and people aren't recommending either.
Both companies are in trouble financially because they pay talk show host too much money, and the merger won't help, no it hasn't happened yet. So they're removing channels and cutting back in order to save money.
Sirius will get you Bubba and Howard, but after you listen to them for a while you don't care. It's the music content I want the most, and Sirius is removing channels.
Keep your money and get an iPhone or something. You'll have more fun surfing the net standing in lines. Sure, the iPhone doesn't have 3G, but it's fast enough for normal surfing.