That's funny, because when I'd drive around the canyons of downtown Toronto, I'd frequently lose my XM (due to the signal being blocked by buildings, I assume) but no problem with the 3G signal. (This was true in my previous car as well -- nothing to do with Tesla's XM radio).
Oh, don't get me wrong, XM is far from perfect. I routinely get several drop-outs on my way to work (once by the library; a single story building, and once going through an underpass; which is made of wood

). I would have much preferred Tesla use Sirius for their sat service. Far fewer drop-outs.
However, when I'm traveling HWY-106 and there's nothing around for hundreds of kilometers but me, trees, and rocks...even XM would be a life saver if I don't have a good audiobook.