With those two resorts, I would definitely be tempted to go without the car. But for us we would have to make SURE we had no interest in a Sweet Tomatoes night, or in going to GiftWorld or one of the other kitschy souvenir places. And since we're ALWAYS in the mood for Sweet Tomatoes, we'd go back to the rental car.
When the Disney transportation is working, it works. When it doesn't, it's miserable. My aunt and husband experienced a perfect storm of nasty transportation problems once, and they haven't trusted the system since. The dispatch system had just changed, and they waited well over an hour at DHS waiting for an OKW bus. Others who came behind them not only missed their Olivia's reservation, but didn't even get to OKW before Olivia's was fully closed that night. It was just a cruddy situation. I'm still not sure why we didn't just hop in the car and get them, but we were all brain-fried and not thinking. DH and auntie are high-stress people who lose the ability to be clear in such situations, as well, so everyone just kept waiting. It might take the same 15 minutes to find a car as it does to wait for a bus (though I've waited longer for buses overall), but it's NEVER taken me 2 hours to find the car and drive to the resort.
As for a cool car...assuming you have a nice almost new rental car, that AC will be whooshing in absolutely no time, and by the time you're out of the lot it will be freezing in that car.