Renting a car is entirely up to you. Magical Express will pick you up about three hours before your flight departs. You may be able to stay at Walt Disney World a little longer with a rental car than you could if you used DME, but not much. Orlando International Airport recommends passengers arrive at the airport (i.e. in the terminal, not driving into the garage) two hours in advance. You never know how backed up the car check-in line will be - I've been the only person returning a car, and I've been in a line of about fifteen customers. Security lines can be horrendous - sometimes you can get through in ten minutes, sometimes forty-five or longer... and your plane will start boarding about twenty to thirty minutes prior to departure time.