Your question all depends on timing.... sometimes driving can be MUCH faster, especially in the mornings to mid-afternoon. At night, when there are rushes of people, driving can take longer (depending on where you are going). Disney Springs is kinda a zoo right now, but it's gotten a lot better since the Orange Garage has opened up.
Also, since I am a local, I know all the tricks, backroads, and my way around. For a tourist driving can be MUCH more stressful... dealing with GPS directions and unfamiliar territory. Take that for what it's worth.
The reason I always rent a car and drive when going on vacations (at least for most places, including when I lived in Seattle and used to visit WDW) is because I like being in control. For example, I like knowing that if I miss a plane flight, it's my fault and not someone else's that I can get mad at. But it's up to you if it fits your individual personality or not.
For most people I would recommend using Disney's transportation options, especially if you plan to stay on WDW property the entire time you're in Florida.
Ethan