2minny:
There's no catch... we used Tiffany last August and loved it - we will never use Mears again! You've figured it out correctly... it is more expensive for 6 people to take Mears than to get a van from Tiffany. They have quoted me $95 for a van to WL (that's round-trip) and they have a $5 off coupon on their website or if you have a passporter, they have an $8 off coupon in them. It's a great deal, especially if you take advantage of the free 30 minute grocery stop (we bought breakfast stuff, and a bottle of liquor and that in itself saved a bunch of money).
It's also pretty cool to come down the escalator at the airport and see the guy standing there holding a sign with your name on it.
The only extra thing you'd have to think about is tipping the driver for loading and unloading your luggage... we always tipped the Mears driver, so this part didn't change for us.
Now that we've used Tiffany, we would never go back to the shuttle - it wastes too much time and they cram the people in there like sardines.
Enjoy your trip whichever way you decide to go - but be warned, once you use Tiffany, you'll be too spoiled to go back to using Mears!