I have to agree with the Tiffany Towncar suggestion. How many trips to Universal are you making? How are you getting to and from the airport. The reason I ask these questions is this is a decision I had to make as well. It looks like it will actually save us money to rent a care at the airport for our length of stay rather than use Tiffany.
We are planning two days at Universal (about $60 each trip) and then to get to and from the airport was (50 - 60) around $180 then figure tips, minimum would be $30 because you KNOW you will get GREAT service. Figuring it this way it is about the same to get rental. We have gone to Disney many times and never had a rental car so we shall see.......
We used Mears many years ago to go to Universal and were very disappointed. This might have changed but they would only pick us up on their schedule. I remember there was a show in the evening that we really wanted to see but could not because Mears would not pick us up later than their sceduled time. It was a very disappointing thing for us. We just figured you are paying them for a service and they should tailor something like that to your needs.