Let's say the return flight is at 6:00pm..you scheduled it that way so as to have some park time on your last day. If you use DME you will be picked up around 3ish. You may, or may not, make subsequent stops. In over 6 trips with DME, I have only gone directly to MCO from my resort one time. Let's assume there will be at least one stop. So...with that in mind...you should get to MCO by 4:00...just the amount of time MCO has stated you leave for checkin. Of course, if you have already checked your bags, you don't need to worry about that. So....you have about 90 mins to kill now. Most airlines start boarding about 30 mins before departure time.
If you use a car service, or a cab, you could conceivably leave your resort about 3:45-4pm...gives you about an addtl hour. I figure that if you need to be there, at the gate, by no later than 4:30. But others will say you don't need to be at the gate until one hour before your flight. For me...that's cutting it way too close. Security could take you much longer than you figured on, there could be traffic along the route, maybe an accident..who knows.
So, you may get an extra hour at the parks if you use a car service or a cab. It just isn't worth the expense to me. I suppose that if you have checked your bags that morning, with RAC, and have only small carryons, then you might be able to get picked up right at the park...but that's just nothing I would do. I hate having to use a locker and some bus drivers won't let you on the buses if they think you have 'luggage' with you.
You do need to be back at your resort about 30-40 mins before your pickup time. You should plan on it taking 10 mins to get your carryon bags from Bell Services, then you should be out and waiting for either your DME bus, or your car service, 15 mins before your pickup time.
To me, spending an addtl $60+ in order to spend an addtl hour in a park is silly..but that's just my opinion.