Rent the car!!!
We have been to Disney many times and never rented a car till this trip. Wow , it was terrific having it. We have 3 kids (11,7,3) and it was great not having to wait for the bus or carry sleeping kids at the end of the night, or folding up the stroller, carrying all the packages from the day, standing room only at the end of the night, all those stops at different hotels. We did US for two days. By the time you added up the price of transfers to US, plus town car to and from the airport (absolutely refuse to take mears'-too many stops before you can get to your hotel after a long flight, plus the pick up on departure day is scheduled way earlier than your flight) and including tip, it ended up being the same price to rent a minivan for the whole week.
I was worried we would get lost, but the signs are pretty good,& I studied up on an orlando map ahead of time. There was only 2 times we got a little mixed up on which way to go, but that was off-property and really no big deal. US is a straight shot on the freeway for WDW, signs everywhere.
We stayed on site for 7 days and never once got on a Disney bus, that's how much we liked having a car. Parkings free for onsite guests and the parking lots not that much farther from where the buses(from the value's) pick up and drop off
Really check around for good deals, rates change all the time. Look at mousesavers.com for discounts. And sign up the for quick registration program at whatever rental place you choose. Dollar has Fastlane, another has one called Emerald Aisle , can't remember which company, but they each have pre-registration which makes picking/dropping off car real easy. And rent from one of the 5 on-site at the airport, it's really convinient that way.