Regardless of your choice (wheelchair or
scooter - Disney calls scooters "
ECVs") remember that the *average* Guest at WDW walks between 3 & 10 miles
per day.
Very few people can sit down in a wheelchair and self-propel through one "normal" day at home before their arms/hands/shoulders give out. It takes time to build that kind of stamina and endurance, unless you are a highly trained athlete in top form. You don't want to spend the rest of your WDW trip nursing sore arms & shoulders... the idea is to have fun and LESS pain!
Make sure you have a volunteer "pusher" ready if you decide to rent a wheelchair.
A rollator is another great option you could consider - basically it's a lightweight walker with a built in seat (and typically, a handy basket under the seat) that you can walk and push for stability, and when you have to stand in line, or just need a break, you can plop down on the seat.
ECV's don't have to be parked and walked away from - you can take it almost anywhere at WDW (including right up to the load platform for a lot of the rides). This includes buses, the monorails, and the majority of the boats - the exceptions being the small launches that turn to/from Poly & GF to MK, and every other boat at WL/FW.
If you have specific questions about using one device versus another, perhaps we can help you sort out your options. You can always arrive at WDW, start with a one device, and if you don't like it, call the rental vendor and ask them to bring out a different type of device. I don't recommend it - you will be losing precious vacation time making the swap! But it might be the best way to go if you aren't sure what is best for you.