We have done both no car and car trips. For us, the convenience of the car outweighs the cost. I hate waiting for the bus and then dealing with the crowds, whether you'll get a seat. With a child you have to fool with getting the stroller on & off the bus (yes, you have to with a car but you don't have to deal with impatient, tired people who just want you to get on the bus) and you have to do this on the trams, too.
If you like to resort hop (which we do), car is the best way. Otherwise, you take a bus to DTD or the parks then transfer and vice versa. It can take you a good 2 hours to get from one resort to another depending on how long you have to wait.
Plus, a bus has to make multiple stops within the resort and some stop at the water parks too. Many times, if you stay at an Epcot resort, you may have 5, yes 5 stops. Boardwalk/Beach Club/Yacht Club/Swan and Dolphin.
You will kill a lot of time with the buses. But we do take the bus to the MK. The bus lets you off near the entrance. If you drive, you have to park, then tram to TTC, then ferry or monorail to MK. The bus is definitely easier at MK.