I've taken almost every possible type of transportation from MCO to WDW....rental car, car service, DME, Mears shuttle (both the old and the new). I have stopped for groceries with the car service, I have had groceries delivered, I have gone in a rental car with a friend to get groceries.
So....I voted DME with grocery delivery. Why??? I love not having to wait for my bags at baggage claim. I want to get started. I usually make it to my resort, via DME, in about 70 mins...from gate to resort. I have had times of as short as 60 mins!!!
With a car service, if you have carryon bags only, it will take you about 45 mins...gate to resort. But, if you have checked bags, add at least 15 mins to that time. Oh, and $100 r/t for a car service??? Most run a bit more than that, and don't forget the tip of about $20-25. And you have to add on the time it takes you at the grocery store....so if you land at 5:30, and use a car service, you won't get to your resort until about 7-7:30, depending on how long it takes you to deplane. Don't forget....landing time is NOT the time you get to the gate...add another 10 mins for that.
IF you take DME, you will most likely be at your resort by 6:45-7, and your groceries will be waiting for you. Yes, there is a delivery charge but it's only about $15. Bell Services will bring the groceries to your room for you, or you can take them yourself.
It all depends on what kind of experience you want. Yes, a car service is a bit faster (not including the grocery stop of course) but I don't mind spending an addtl 20 mins and saving that $100+ and spending it elsewhere at WDW. But I like the DME bus. But there are times when I want a nice towncar...just depends on the trip.