Service: The towncar is better. Only your party will be in the car, so you won't be stopping for other folks at their resorts, or waiting for the other folks to fill up the shuttle before you leave the airport. Think of the shuttle like a small bus.
Price: The towncar is priced per trip, the shuttle per person. I've found that 3 passengers is about the break-even point. If you have 4 passengers, the towncar is probably cheaper. For two passengers, the shuttle is probably cheaper.
We (3 of us) usually use Tiffany Town Car. They are always waiting for us at baggage claim, and whisk up directly to our hotel.