Depending on where you're parked and what time of day it is, and where you plan on finishing up, and which park closes latest, it's a toss-up between driving yourself or taking the bus.
You can also walk or take a boat (or both) from International Gateway in the World Showcase, which isn't bad (walk to the Swan/Dolphin and pick up the boat to MGM there).
Either way, it could take up to 30-35 minutes.