I have used Tiffany Towncar before, and will be using them again in June. However, I haven't had a group bigger than four, so it is always just the towncar type vehicles. I can recommend Tiffany, as I received excellent service from them. You can also check out their webpage. (not sure if I can link it here since they are not a sponsor, so I won't)
You can also look at the main DIS webpage, as they have a towncar/van service that is advertising on the site.
My opinion is that you will be in a van with 6 people (or a limo if you splurge!!). Also, if you don't plan on leaving disney property during your trip, then the towncar service is worth every penny IMO.