We always use Tiffany and the benefit of a pre-arranged town car over a taxi is this:
With the town car service, when you arrive at baggage claim there will be a neatly dressed driver waiting for you, with your name on a sign that he/she is holding up. The driver will retrieve your luggage for you, load it in the car, and whisk you to the Swan. The driver will be extremely knowledgeable about WDW and will chat (or not) depending on what you desire. The vehicle they are driving will be appropriate for the size of your party -- I use the term "town car" but the service will provide a van that's also neat and clean.
If you want a taxi, you get your own luggage, schlep it to the taxi stand, and you get the driver you get. The size and cleanliness of the vehicle is luck of the draw (and if there isn't a van waiting in the queue you might need to split into 2 vehicles which then doubles your cost, or wait for a van to get there).
On your return trip to the airport, with a town car the driver will be there at the resort, waiting for you, at the pre-determined time. This person again will take care of your luggage with a smile (unless your airline participates and you decide to use the program by which you can check your luggage for your flight directly at your resort).
You can see why I'd go with the town car service every time!