I've used both Mears and Tiffany towncars and both are very good companies. The good things about Mears: reasonable prices, easy to reserve in advance but also just as easy to get tickets at the airport. There are a couple of drawbacks to Mears though: can be long waits for shuttle, stops at several hotels add time, pickup for airport return is a very generous time window, so you will have to leave very early if you have a morning flight. (We had to leave at 4am for an 8am flight.)
Tiffany is, IMHO, a better choice if you have more than 2 people in your group. The price will then be comparable to Mears and you can go right to your hotel (unless you would like a grocery stop, which they will do.) The town cars are roomy, comfortable and spotlessly clean; drivers are courteous and careful. You'll still have to leave a couple of hours before your flight but because the car is picking up ONLY your party it won't be quite as early as with Mears, which has to make other stops.