As a rule of thumb, the relative cost of a town car vs. Mears is a wash at three people and you come out ahead with a town car starting at four people. Mears charges by the person, while town cars, limos and vans are charged by the vehicle no matter how many people ride.
The other advantage with booking your own town car, limo or van is that you'll go directly from the airport to your hotel (and vice versa) without stopping at other resors. In addition, you set the schedule for pick ups and don't have to wait at the airport until the Mears bus/van is full or catch a bus four hours before your flight to fit Mears' schedule.