Only you know how much money you have available to spend. Ahead of everything else, I respectfully suggest that you don't spend money you don't have.
With that disclaimer out of the way
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, which hotel really depends on what you intend to do for those extra 2 days.
If you intend to stay at the parks all day and treat the room as a place to shower and sleep, then go cheap.
If you want to slow down and spend time at the hotel, then go with the Yacht & Beach Club (Y&BC).
A few random thoughts:
Disney currently is offering a 30% room discount for the Y&BC (plus one more ticket day free) for May when you add a minimum 4-day ticket.
The 2 days you are adding offer you an opportunity to try a hotel that you normally would not stay at. It's just enough time to get a flavor of what the hotel is like without breaking the bank.
In Orlando, May is a very warm time of the year. Presumably you'll want to spend the afternoon poolside. The Y&BC's Stormalong Bay offers a unique poolside experience.
If I was doing Y&BC for 2 days at the end of my vacation, I'd get my fill of MK, DAK, and Epcot's Future World before those 2 days and then focus on DHS and Epcot's World Showcase for those 2 days. This includes (for example) spending a few hours in the morning at DHS, heading back to the hotel's pool for the afternoon, and then spending the evening in Epcot's World Showcase.
If you and your 23 year-old nephew are so inclined, staying at one of the Crescent Lake hotels (Y&BC, BWI, Swan, or Dolphin) allows you to experience the World Showcase as adults (i.e. drinking), with just a short trip back to the hotel at the end of the evening.
Spend some time on the Boardwalk, across Crescent Lake from the Y&BC. The Boardwalk is the most fun after dark, so maybe this is something to try after Epcot closes at 9 PM.
Please let us know what you decide.
