Typically, an out of state purchase where less or no "sales tax" was collected by a dealer will require you to remit a "use tax" equal to the sales tax you would have owed your resident state if you bought it in state. I believe the same holds true with regard to differing rates within the state, so you will most likely be out of pocket the same amount either way.
However, the FL county surtax is typically only charged on the 1st $5000 of the sale. So if you are buying a car in a 7% county, you will only pay $50 more than if you bought it in a 6% county. For that kind of money, I'd get the car where its most convenient for you.
FL Dept Rev - Florida Tax Information