We've done both. To me it depends on the age of your kids, when our youngest was a toddler we did the buses and I would do it differently if I were to do it all over again. Our last trip down to Disney we had a car and it was heaven leaving the parks at night and bypassing the long lines that waiting for the bus.
In two weeks we're going down and are not getting a car but our kids are a little older now so we're hoping it will go a little smoother.