I'd think that XM Radio is ubiquitous enough by now that people wouldn't care about stealing it. Kind of like how 25 years ago radar detectors were a hot item and people would hide them away but no one cares about them now. I see them more prevalent now in cars than a few years back but they don't seem to be big theft items.
I think the big theft items now are GPS, fancy phones, and iPads. Leave those out and you increase your chances of getting broken into.
Now, if you're still concerned about it you can probably just place the antenna on the dash right up against the glass or put it on your back deck against the glass. This won't be as good for the signal as just mounting it to your roof but you should still be able to do it. I'd play with it on my dash first and make sure you can get a signal before taking everything apart.
I mounted an XM antenna under my dash and it worked well. I had a few dropouts so I just mounted one to my trunk (it was easier to just buy another one and put it on my trunk than to take apart the dash). I think the one mounted to my trunk gives me a little better performance. It's a tiny little black dot, though. You really don't notice it.
Between XM and Sirius -I'd say go with XM and get Best of Sirius (look for other threads here). Basically you get more bang for the buck and it looks like that's the way Sirius/XM is headed, anyway.