I agree, use your own car. I can't imagine spending even a minute of my vacation waiting for a bus. When we are ready to go to AK or MGM we just drive our car.
On the other hand, you may be one of those people who prefers having someone else drive, and are looking forward to not driving on your vacation, or if you are the type to get "freaked out" driving in an unfamiliar place (though we find it incredible easy to find our way around WDW) you might prefer the bus. Some folks say they find it more "relaxing" to not have to even think about their car while in WDW, and love taking the busses. For me, I find it more relaxing to drive our rental car wherever and whenever we want.