We are with beattyfamily - we always rent a car. The flexibility is wonderful. WDW buses are ok for going from your resort to a major park. However, getting around otherwise on the buses can be a challenge. A typical day for us involves leaving our resort, having breakfast at another resort (GF, BC, etc), spending the morning in one of the parks, going back to our hotel for a swim/nap, or perhaps to a water park for a few hours, shower and change at the hotel, off to dinner (perhaps in a park or another resort) and off to another park for the evening. You just can't do that with the bus system. We are DVC members so we like to stock the fridge with some sodas and goodies and the car is great for getting to the supermarket. Once our DD got sick and we had the car to go to the clinic. Buses to DTD/PI can be a long ordeal. The ride back to your hotel in a wet bathing suit after spending time at a water park is much more pleasant in a car than on an air conditioned bus.
Talinthor's comment about a car with young kids is very true. We have little ones (3 yo and 10 mo) and a car is a BIG advantage. They can fall asleep (afternoon nap or pooped out at night) in the stroller and we can leave them there until we get to the car, put them in a comfy car seat, drive to the hotel, and get them in bed without really waking them. Using the buses you'd have to wait for a bus, get them out of the stroller at the park, get yourself, the kids (possibly still sleeping), diaper bags, backpacks, any other packages, a folded stroller, etc. onto a very possibly crowded bus for a ride that, while short, will seem like an eternity. Once at your hotel you have to get all your stuff off the bus, along with the kids who are now awake and cranky, and get to your room. Not for us, thank you.
Can you tell we are not big fans of the buses? Only you can decide if the extra cost is worth the flexibility, if you think you would use it. For us it is and a car is always in the
budget for our trips.
Good luck with your decision. It's too bad cars are so much more expensive this year than they were last year

.