Having used all three methods, i.e. windshield, vent and dashboard nonfriction mat, my preferred is the vent, followed by the mat. However, the pros and cons are personal and depends on your needs.
For me:
Widshield
Pro:
use of standard mount which comes with TT;
in line of sight when driving
Con:
can fall off,
leaves sign of GPS on the windshiled;
may block view;
illegal in some states
Non-friction Mat
Pro:
can be easily placed in different spots on the dashboard;
can be put away without too much of a problem,
in line of sight when driving
Con:
potential for movement or flying back in a very sudden stop or accident;
may be illegal in some states;
cannot use the standard mount that comes with TT,
too short to attached;
may block view
Vent
Pro:
does not block view; legal in all states;
not easily visble to others
Con:
block a/c air;
depending on kind of vent model may move about a bit and require both hands to insert and remove;
depending on model, may not be quick and easy to move to another car
Bottom line, it all depends on what you are looking for, and as in selecting a GPS it is not just one aspect of the product.
Others, feel free to add to the pros and cons of my list, because a good mount is as imporatnt as a good GPS. In my case, I also had a Navigon 7100, which I sold after using it a few times, primarily because I did not like the mount, which to me was too big and visble