We always rent a car for our trips to WDW and always drive to the parks and DTD.
Driving around WDW is very easy and parking at the parks is free since you will be staying on-site. We just like the convenience of not having to wait for a bus (or two), and possibly standing on the way back to the resort, plus if we want to go to directly to another park from the one we're currently at, it's much easier than having to take the bus to a resort, then take another bus to that park.
Parking at the MK is near the TTC, so you will need to take either the ferry or monorail over to the park, whereas if you do take the bus, it drops you off right at the MK. Parking at the other three parks is very easy.
Parking at DTD can be a bit of a pain at times depending on the time of day and day you go, as sometimes the lots are full. We've had to actually circle the parking lot a few times to find a spot on occasion or had to park way over by Cirque De Soleil.
It's really up to you what is best for you and your family, but driving to the parks is our preference..