I would guess that a rental car might save you a few minutes, but the actually among of times depends on a lot of variables.
For DME, it might take longer because DME may stop at multiple resorts before getting to your resort and you might have to wait for the bus to depart MCO. I generally allow about 90 minutes from when you de-plane to when you arrive at your resort (assuming you head straight to DME from the plane). Another common estimate is to allow about three hours from when you de-plane to when you're walking into a theme park.
If you rent a car, you have to factor in the time to get your luggage from the carousel and then getting the rental car. Drive time would be the same (probably), so then it's a matter of unloading your luggage and parking the car at the resort. I've not used a rental car to get from MCO to WDW, so I don't know how long to estimate it takes on average.