We always rent a car, but that is mostly due to the 5 of us would rather sit in the comfort of a rental car than try to squeeze into a crowded bus. On the bus we rarely sit together, since it is rare that there are 5 seats next to each other. There is one benefit of the bus -- the bus terminal at MK is much closer than the parking lot, and you do not need to go through the TTC. However, my family loves to ride on the large ferry that goes from TTC to MK, so if you ride the bus, you never get to experience that (unless you take a round trip on the ferry).
One way we save a little money is by having my wife and 2 of my kids take DME from the airport, and all of the luggage goes with them on DME. Then my other daughter and I will rent a much smaller car, since we don't have to worry about putting all of the luggage into our car along with 5 people. This significantly reduces the rental car price. Also, my daughter and I will stop at a grocery store and get milk, cereal, water and other items to have in our room to save a little money on overpriced waters and breakfasts. Then on check out day, we check our bags at the hotel in the morning, and we can store all of our carry on bags in the car, so at the end of the day, we can just walk to the parking lot, get in our car and drive straight to the airport. The DME requires you to go back to your hotel, and leave 3 hours prior to flight departure... by having a rental car, you can stay much later at the parks.
One year we did have a little issue.... we rented a car that we all remembered as being silver, and went straight to DHS. When we left DHS, we got to the lot and there were hundreds of silver cars that all looked very similar, and we couldn't really remember the type of car that we rented. We roamed around for about 30 minutes clicking the panic button on our car key until we found it.... The bus probably would have been better that day...