We are taking Prime Time Shuttle. We have a party of 5 going and they are giving us a private van for $67.00 each way. This way no boarding, shoving to get on a motorcoach with 50 million others. I thought it was great. I did a mapquest mileage inquiry and I thought it was more like 20-30 miles....I could be wrong. I hope that helps.
I suggest that you buy a transfer from Carnival (through your travel agent or directly). It saves a lot of money and is much less a hassle. You board the bus in baggage claim area and you and your luggage are transfered right to the pier.