Hi, Tammi! To determine if you are better off renting a car vs. using your resort's transportation, you will need to figure in all the costs of each option.
If you do NOT rent a car, factor in transportation costs to and from the airport as well as travel to the parks.
If you rent a car, you need to add the price of the car rental, gas, and daily parking at the parks (about $12 per day, subject to change.) Parking at Downtown Disney and the water parks is free.
When we stay off property, we always rent a car. It is just easier to get around to the parks and for traveling to and from the airport. Before renting a car, visit
www.mousesavers.com and look-up some rental car discount codes. (Get some quotes with and without the codes, it is sometimes cheaper to NOT use a code!) We like to rent from Dollar. Their cars are right at the airport, so no need to take a shuttle. Also, by renting a car, you can come and go on your schedule, you don't have to wait around for your hotel bus.
Good luck with whatever you decide!