I always go with a rental car, because I usually can get a rental rate that is very comparable to the cost of the shuttle or bus (family of 5) and the convenience is far better too. Every trip I need to use the rental car for trips to Target, restaurants and even visits to other sites or parks.
I went with the bus service way back in '97 (without kids) and it was nice to be able to kick back and have a "tour guide", but I was on someone else's schedule. The cost wasn't bad either... But I wished I had my own wheels...
And that is my ten cents!
