Rent The Vehicle. Don't forget that if you are using the Disney buses you will have to fold the strollers and actual seats on the buses are few. Also, we consider riding the parking lot trams and monorail to be part of the fun: When else are you EVER gonna get to ride a monorail? (One trip we were staying at ASMusic, using the bus to the parks. DD asked one day if we could ride the monorail just to ride it- the bus drops you AT MK, so no monorail for us- but of course we rode it, it's part of the fun!).
If you are staying at a Disney resort hotel, your parking at the hotel AND at the theme parks is free.
We've never had much trouble with the traffic leaving the theme parks. I mean... yes there IS traffic and sometimes a line to merge from the parking lot onto the street, but if you think that's a pain, think about standing in line after a long, hot day of standing in lines in the parks, waiting for your bus, but maybe you don't get on the first bus and have to wait for another. I DETEST standing in line for a bus at the end of the day, and then to be packed in shoulder to shoulder with a whole bunch of sweaty, tired, cranky strangers!
You can find good (relatively good) deals online using carrentalsavers.com or autoslash.com, and once you have a reservation number autoslash will stalk it for you, sending a message if they find a better deal on the same vehicle class/size. Stick to the major rental agencies, though. The smaller ones (the ones that are NOT at the airport) are cheaper but the horror stories about renting from them are legend... and NOT in a good way!
