My opinion, as someone who has used both DME and car services many times, is to save the money and take DME. If you don't have TSA Pre-Check, then the 3 hour timeframe to get to the airport, check luggage, and go through the typically awful MCO security without feeling stressed or rushed is pretty spot on. If you do have TSA Pre-Check and you also want to leave the resort a bit later, you can put a different flight time down (about 30 minutes after your actual flight time) when signing up for DME. I wouldn't go further than 30 minutes, though, since you're checking luggage at the airport, and that can be a cluster of its own.
Even using a car service you have to allow plenty of time, as you'll probably want to arrive at the airport 1.5 hours (with Pre-Check) or 2 hours (without Pre-Check) before the flight, and it takes 30 minutes to drive directly to the airport. And car service & Uber drivers are occasionally late, aren't free, sometimes play their personal choice of music or news on the radio (or sulk if you ask them to turn it off), which can take you out of the Disney mood, etc.
Magical Express is about to disappear forever. Why not use it while you still have the chance?