We haven't tried manual to compare, but
ECV has been a trip saver for us.
DH has 3 fused lower back vertabrae and can't walk more than 2 miles/day. After
renting a scooter in Orlando for 2 trips, we opted to buy a used $150 scooter off Craig's. It lasted for 4 week long trips and 4 RT flights. It still works but the key snapped off.
At first we were worried how the
ECV would impact the trip but it really doesn't change much for the party except making WDW life so much easier for the user. DH loves it because he's able to keep up and do everything he wants. Here's a list of what to expect at different rides (which most you can ride the line and transfer when it's your turn to board):
https://disneyworld.disney.go.com/guest-services/mobility-disabilities/WDW is fantastic and has it down to science. So helpful, it is virtually seemless. They'll have it waiting for you when you get off the ride.
The airline JetBlue has been great as well. When we called to ask about flying with our ECV, they bumped us up to 'More Legroom' for free (just the first trip). Every trip DH rides the ECV right up the ramp to the plane and the crew take ECV's and strollers to store last so they're the first thing out and ready for use as soon as the plane lands, no extra fee.
One trip his Mom joined us and we rented her ECV due to knee problem. Having 2
ECVs seemed like it might be hard to manage but our experience really wasn't different than when we used to all navigate by foot. I don't know for sure (having not tried manual) but we feel the 'splurge' for powered is well worth it

Have a great trip!