Can't comment on TomTom, as I have Navigon.
But it works just fine with a dash mount, so should work even better with a windshield mount.
"Small town" is irrelevant. The "assisted" GPS is only for startup. If you are somewhere where there is no data service, it will take longer to get an initial fix, but otherwise no difference. The maps are all on your iPhone.
Now, GPS reception varies according to your location. Has nothing to do with small town or big town (other than buildings in a big city). It has to do with what part of the country you are in and where the satellites are at a given time.