Spydy...you might not have as much trouble as I did with the reception...there seems to be no real rhyme or reason to it, and with the larger Garmin XM puck, perhaps it's stronger? Only way to know is to try it and see if it's acceptable to you. As far as not missing what you've never had...never had a GPS before either, but still knew I needed/wanted one. XM is just so nice, it almost eliminates the need for any other source of music on board the bike. I still have an MP3 CD in the radio, and the MP3's on the Garmin, but the XM just offers up so many different things, and some tunes you haven't heard in a long time, but you'd never think of buying them.
I have found through trial and error, the antenna works the same either on top of the GPS (like mine), in the down fairings or under the bat wing. In my truck the antenna sits in the console, sideways.
The XM still receives fine with the bike in the garage and the door closed, and my door is a aluminum skinned 2" insulated door.
Remember, GPS has 26 satellites (24 operational, 2 spares) and XM has only 1 or 2. So there are areas where you will have no coverage, even on the top of a mountain is possible to have minimal coverage.
XM still has too many "XM" commercials for my taste, but it beats the poor reception of the factory radio.
I also use my iPod on the bike, about 50/50 between XM- iPod.