Magical Express rarely takes as long as the above poster indicates; it's more in the 60 to 90 minute range - which is comparable to how long it takes a renter to collect luggage, complete the rental transaction, and get to the resort. And no DME user ever needs to wait in the room for checked luggage.
But if overall speed is your only criterium, renting at the airport is faster because it's one activity instead of two: rent a car, vs. get transported to WDW then rent a car.