This most likely will not help you to decide, but we do both.
If we are staying at the Boardwalk area, we usually do not get a car. When we stay at OKW we like to have one available for use during the evening.
Getting to the parks or DTD by bus is no problem, imho, just sit back enjoy the ride and people watch. You may have to wait for a bus, but you are on vacation, relax, spend quality time your gang. I also think it is actually easier to get into and out of the parks (especially MK) by bus than taking your own car, And if your really in a hurry to leave the park, then hire a taxi that one time, its about 10.00.
Now a car is great if you want to visit other resorts or have dinner reservations at another resort. We stayed at Boardwalk in April and had reservations at Trails End Buffet one nite. had to take a bus to DTD, then bus to Wilderness Lodge, then another bus around the corner to Ft. Wilderness, took about an hour. So if you need to go from one resort to another it can be kindof a pain without your own transportation.