If you are staying off-site I would highly recommend a car. The one time we did so without one, the claimed "excellent" transportation was not, often more than an hour during the day for the next bus to come. Driving to the parks is much easier from off-site. Moreover, the off-site resort is not interconnected with everything like when you stay in WDW and thus if you want to do anything other than something at the resort, you must have a car.