I think we rented one for my mom from
Buena Vista Scooters (or something like that). We have always rented off-site and found it much more convenient. Also, not a lot of people know this, but when you go to the parks go to "city hall" or the equivalent in each park and tell them you have someone in your party using a scooter. They'll give you a pass and also a book that tells you where all of the handicapped entrances are.
The book is the WDW Guidebook for Guests with Disabilities and there is one for each park. There is a link to the text for each park's guidebook in the disABILITIES FAQs thread, in post #3 (a link to the disABILITIES FAQs thread is already posted in this thread several times).
AK and the Studio were built with what are called "Mainstream Lines". This means the regular line is wheelchair/
ECV accessible and people using them wait in the regular line with all the other guests.
MK and Epcot have converted most of the new or renovated attractions to Mainstream access, but a few attractions were not able to be converted and do have a handicapped entrance. You do not need any special card or pass to use the handicapped entrance with an ECV or wheelchair; the CM at the attraction will see the wheelchair/ECV and route you to the accessible entrance.
The 'pass' mentioned is called a Guest Assistance Card and there is more information about Guest Assistance Cards in post #6 of the disABILITIES FAQs thread. It says right on the card that it is not meant to shorten or eliminate waits in line. Most people using a wheelchair or ECV don't need a Guest Assistance Card unless they have needs that are not met by using the ECV in line. The card is meant for advising CMs about needs that are not readily visible, so someone with additional needs might be helped by a Guest Assistance Card.
And, by the way, it is unsafe to have children riding along on any ECV. All they have to do is accidently hit the control lever and 400 pounds of scooter plus riders can smash into someone or a wall at up to 5 miles per hour.

It is not safe for children to ride on an ECV. I have seen a number of really close calls where the child was pointing at something and hit the driver's hand, pushing the scooter out of control.
The rental agreement from most of the companies actually has a clause that says the renter agrees to not allow anyone younger than 18 drive the scooter and not allow any passengers. WHen you sign the rental agreement, you are agreeing to that.
If you google the instructions/owner's guide for any scooters, you will find a caution not to allow any passengers. Besides the dangers, I would be concerned about liability for doing something that is listed as a caution by the manufacturer.
The resorts
may have wheelchairs to loan, but there is no guarantee there will be any when you get there or that any they have will be in good condition. People recently have posted the ones they got were not in very good condition.
For which is better, a wheelchair or ECV, there are a lot of factors to consider.
An ECV gives a lot more independence and no one will have to push it, but some people don't feel comfortable using an ECV and are willing to give up some independence for not having to worry about steering.
For your mom, some things to think about would include:
- Does she have any problems with vision that would make it difficult for her to drive an ECV?
- Does she have any problems with concentration that would make it difficult or more tiring for her to drive an ECV?
- Does she have any numbness or weakness in her hands that might make it hard for her to grasp the controls and drive the ECV?
Hope she has a good time, whatever you choose.