I have the rugged ridge spare tire mount. It's a great mount but a bit of a pain while dealing with openings the rear. I'm running a 3' firestick II and I'm getting a 7-10 mi range with my Uniden 520 out of the box.
If you go with the license plate mount you will need a longer antenna to get about 12-18" above the jeep. That would mean more whipping around. A pro and con of both.