I think for $250 you'll get quite a lot of value.
- The resort is a deluxe, which means more amenities. Unfortunately, it doesn't mean a larger room if you choose WL. If you chose Poly, it would be a much larger room.
- Access to 2 full-service restaurants and quick service (though this is more limited than the QS at the moderates).
- Access to more pools.
- Ability to take advantage of water activities like boat rentals or view the Electrical Water Pageant.
- Hotel style versus motel...rooms are off of interior corridors, no walking outside to go get your morning coffee, etc.
- If either of your parents has a handicapped placard, you will be able to get free valet parking, which is a great benefit if walking a lot is an issue. (though valet parking is one of the best deals at Disney IMO and if you are dining at other deluxe resorts, its value really pays off...you pay 1x per day and that gives you valet privileges at all resorts with valet parking)
- Boat service to MK is great IME.
- Nicer public areas...huge, impressive lobby, pool bar.
Poly has never appealed to me (I've stayed at every deluxe except Poly...in fact, the only resorts I haven't stayed in are ASSports, POFQ, Poly and OKW) so I probably wouldn't stay at the Poly UNLESS the larger room was a dealbreaker (and it's fairly significant in size difference IIRC). The Poly has the monorail advantage, but I've never found it to be faster than boat service unless you happen to get lucky and monorail arrives just as you get to the station. But it would get you 2 parks without having to use bus service, which might be worth it to you.
It's a shorter ride to Mk from Poly whether you take the boat or monorail. We waited a long time for the boat at Wl. There is always going to be luck involved when transportation is involved, but all things being equal the monorail is faster.
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