This is sometimes a heated argument (at least in New York) between the Howard (Sirius) fans and Opie & Anthony (XM) fans. I have XM. I think the music selection for my tastes (nu-metal, heavy alternative, etc.) is great on XM. XM's equipment also seems to be a lot smaller and well designed.

XM had the MyFI for about a year now which is a walkman-like device that can receive and record up to 5 hours of satellite on the go. Sirius just released their portable and it cannot receive satellite while out of its dock and its recording capacity is only 1 hour. Sirius' in-car units are also extremely large compared to XM's.

I was in a store this weekend checking out the Sirius Sportster Replay my friend bought his wife. The unit on display was throwing off a ton of heat. It's also huge compared to the XM Roady XT.

It seems like XM is the better company in my opinion. This is based on stock value, their equipment, and their unwillingness to throw around millions of dollars to acquire talent that probably can't return the investment (Howard Stern ($500 million), Martha Stewart ($30 million), and Tony Hawk are most notable).

Please note that I don't have Sirius, so I can't tell you specifics about their service. So, take my opinion with a grain of salt.