IMHO,
It all depends on tow things:
1. How many are in your party
2. How willing you are to be patient.
Last September, there were 7 of us and we used Tiffany. It was less expensive for us to use a towncar service (or in our case, a van).
The driver met us at the luggage p/u, got our luggage, brought the van around, and loaded it.
AND most important for me was this... we were in the van, pulling out and there was a HUGE group of people waiting to get onto the Mears bus... in the heat, who knows if they all got on the bus. We went straight to our Hotel, and we were alone and enjoying the sites! Maybe this is on the bus, too, but our driver let us know when we were close so we could get a video of entering WDW.
We're going back this fall, 5 of us this time, and I wouldn't think of going any other way. Again, this is IMO.