I'm going to have a similar situation on a lot of fronts.
I'm not only an XM subscriber, I am a stockholder, so I require XM in any car I own

. I currently have a Terk XM Commander in my VUE, and am thinking about getting one for the smart - I like the feature set, cleanliness of the install, and the fact that it is built in to the car.
I don't think it is going to be tremendously hard to make a bracket to bolt to the underside of the dash (underneath the switch panel for the locks/heated seats, etc) angled and holding the receiver - it helps that my dad owns a fabrication business, though.
I'm also going to hard wire my Garmin Nuvi - centered on the dash. I'm still going to use the windshield mount as I am going to be parking in public garages/apartment parking lots for a bit, and I don't want a smash and grab going on - plus I will be sharing this with my other car. I've got a spare power adapter, so I am probably going to splice this into the power outlet circuit, run it up behind the clock and tach pods, and have the little USB connector snaking out through there.