If airfares continue to be expensive, we may drive down in October, and yes, we will rent a car. Our cars are only 3 years old, but it's still nice to not add the extra miles and need for another oil change on our own cars. Plus we know we'll be getting a new, reliable vehicle and that if we have any problems we can just call the rental company for a replacement.
If we drive, we factor in the gas, staying overnight and eating out on the way there and back. For just DH and me, it would be cheaper and less time consuming to just fly, but if four or more of us are going, then driving becomes our first choice unless I can grab cheap flights for our dates.
We usually rent a car once we're at WDW to get our groceries, so I factor in that amount to the flying part - if we drive down, then we already have our rental car.