the cost is comparable, it's easy to take a folding wc on transportation or in a car, they are much more comfortable bc they are in excellent condition & adjustable, and you don't have to fuss with renting on a daily basis (my dh has stood in a lot of lines in his time to get the wcs, they run a tight ship but it's very high volume bc it's combined with stroller rental). we've found that the qualitiy of the WDW rental wcs is very variable and you usually cannot adjust anything (like the footrest height). I was there in October and the parking lot temporary wcs were available. it can be a major hike from the lot to park entry plus you have some standing to go thru security and whatnot, so this is a great idea. I prefer to reserve my standing/walking time for something more fun!
all this being said, though, after countless trips using rental Disney wcs, I rented a
scooter (from Walker) for the first time in October. it was just AWESOME and totally worth the $$$.
if you don't rent offsite, ask your resort what they have. if they don't have a suitable wc, they can often borrow from another resort. there are no guarantees, but it's worth asking. if it's not a busy period and you are willing to wait a few days while they work on it and rent on a daily basis in the interim, they may well come up with something decent. I found that the resort wcs were in better shape generally than the park ones but I know I've seen another poster here with a different experience. I used to prefer borrowing from the resort to avoid the walking necessary to get to the parks, not as necessary with the new system of loaner wcs.
note that the Swan (and likely the Dolphin) do NOT have loaner chairs. the Swan rents for you from from offsite if you ask in advance, not sure who pays.
when are you going? if it's over the Christmas/New Years period, please post. there are some add'l tips that may be helpful to you if you do rent park wcs during a super peak period.

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