We are going to WDW with my DBIL who needs his wheelchair as he is paralyzed on one side. We are not sure whether to push him in his manual wheelchair or to take his electric wheelchair. My DSIL thinks he may have trouble with his limited peripheral vision with the electric wheelchair if someone darts in front of him. I am concerned that my DSIL has not really ever pushed him around for any real distance like Disney so I lean toward suggesting the electric wheelchair at the lowest speed. This is DBIL first time at Disney. What do you suggest?
I would suggest the manual (push) wheelchair for the reasons I bolded above.
It is more than just your SIL going with him, so you have multiple people to push, not just her.
Your SIL is worried about his ability to drive safely due to limited peripheral vision.
Peripheral vision would be not only important to consider people darting out ahead of him, but also his ability to manouver in tight places like lines, stores, getting in and out wheelchair accessible ride cars, busses, boats and monorails (narrow ramps). There are other things, like the sides of sidewalks with curbs and curb cuts that may also be a challenge.
Turning to the lowest speed will make any hits less intense (the force something is hit with equals the speed times the weight/mass), but will not eliminate the hits entirely. So, if your family members are the ones walking beside him, they might get hit because he can't exactly where they are.
Some people with vision deficits also have a lot of difficulty maintaining a straight line when driving the wheelchair. This may not be a problem for him in everyday life, but would at WDW, where lines and other things require straight driving.
Slowest speed on a power wheelchair is also not going to be a comfortable speed for most people to walk along with, unless they are all slow walkers. usual walking speed is about 3.5 to 4.5 miles per hour. The WDW rental
ECVs are set to 3 miles per hour and people frequently complain about how slow they are. Slow speed on a power wheelchair is all the way from barely creeping to about 1.5 miles per hour (on an average power wheelchair).
It's his first trip to WDW, so he is going to want to be looking around at everything. Driving a power wheelchair will mean his attention will need to be focused on driving, not on seeing things.
My DD has both a power and a manual wheelchair. She does not have the stamina or ability to propel her manual wheelchair for long distances. She has had a power wheelchair for more than 12 years and is an excellent driver.
Despite that, we have chosen to stick with her manual wheelchair for WDW.
Why?
Even though driving a power wheelchair is not physically tiring, it is mentally tiring. The driver needs to be alert to everything going on around them at all times and be ready to react, sometimes with no warning. A whole day of that is very tiring, especially for someone who is not used to driving the power wheelchair long distances.
Driving a whole day somewhere familiar and short distances, is fine, but she gets very tired driving in unfamiliar places or when it's very busy (like shopping is fine for a day in unbusy time; shopping for part of the day during Christmas shopping time will wipe her out for the rest of the day.
Driving a power wheelchair requires using the same small hand and wrist muscles over and over. My DD gets finger, wrist and hand cramps after driving a lot.
When she is tired and doesn't want to/can't drive herself, we have 2 options.
One is to disengage the motors and put the wheelchair into free wheeling mode to push it. But, an average power wheelchair weighs about 225 to 250 pounds (not counting the weight of the person) and is not meant for pushing. I have a difficult time pushing my 80 something pound DD in her power wheelchair in freewheeling. Plus, in free wheeling, you have no brakes, so the pusher has to stop the wheelchair (very difficult on an incline)
Option 2 is for me to drive the wheelchair. I have gotten very good at it over the years, but it does take practice. Because DD's joystick is on the right side, I need to drive with my left hand. This is OK when there is lots of space and I can walk beside her, but when it's narrow, I need to walk behind, kind of crouch forward and reach over her - driving with my right hand. That doesn't work for long because it gives me cramps in my back.
My DH is getting better at driving it, but me being most experienced means I am the major one driving when DD can't/doesn't want to.
Wheelchair accessible ride cars are another factor. All of the wheelchair accessible ride cars are manual wheelchair accessible for the majority of manual wheelchair (some with very low footrests may have difficulty on a few). Some of the wheelchair accessible ride cars are not power wheelchair accessible - it may be because the ride car has protrusions that stick up (the Magic Carpets of Aladdin) or because the ramp is too steep (Ellen's Energy Adventure).
So, for those that are not power wheelchair accessible, the user will need to transfer to a manual wheelchair. They have those at attraction, but thy are basic wheelchairs without swing away footrests or armrests or seatbelts.
It is possible to fly with 2 wheelchairs. We have never done it because messing with one wheelchair is enough for us, but I know people no have. Tht gives the option to switch wheelchairs depending on what the plans for the day or his tiredness level is.
Another option would be to bring one wheelchair and rent another one from one of the outside vendors. We had actually looked into this, but because of my DD's size and need for some special options, like specialized seating, this would not work for us at all. It may be a good option for someone who has more 'plain vanilla ' needs though.
So, more things to consider. I would suggest discussing all this with your SIL and BIL. He may have some strong opinions about not wanting to be pushed or wanting to not have to worry about driving and being able to see things.
If in doubt, I would lean towards the manual chair because it is a safer choice and gives more options.
I assure you Disney can be hilly, just pushing someone from Haunted Mansion up to Its a Small World would take some doing. After an hour you would have a bad back or hand blisters. I promise a 
