It does depend on what you're going to want to do. If visit the parks, which parks? As this will decide which hotel is likely to be the most convenient.
If you would just like to make use of the hotels facilities (and if you're paying for them you might as well
) then I'd look into which hotel offers the facilities you're looking for. It's very much a personal thing, but if I was going to make use of a WDW hotel for a short stay like this, I'd really want to make maximum use of the hotel. You can visit the parks from any location and yes, the WDW transport/access does make this a little easier BUT you've got 3 weeks, saving 20 minutes on getting to a park isn't, IMHO, a huge factor.
Spa facilities are offered at Grand Floridian and Saratoga Springs. My feedback is that Saratoga is the better of the two.
Pool facilities The Yacht and Beach club have the most AMAZING pool and it's by far the best on site. Of the others I liked AKL as it's a pretty good pool, but I loved being able to go for a little (20 yards) wander and looking over all the animals on the savannah. Of the moderates I though Coronado had the best pool facilities.
recreation activities Many of the hotels have access to water sprites (mini speedboats) that you can rent, these look great fun. I know Y+B, BW, OKW, PO, CSR have them I'm sure a few more do.
Restaurants The GF probably has the best restaurants on site, but they may be a little over the top for the 6Y old ( you do have the option of taking the resort monorail dropping the kids in the neverland club at the Poly and enjoying a special meal alone, but if you're only there one night you may prefer to stick together. My kids would MUCH rather have gone in the Neverland club than gone to a stuffy restaurant LOL) The Y+B and BWI offer you walking access to either EPCOT (tix permitting) or the Boardwalk area of restaurants The Y+B has some excellent restaurants of it's own). OKW and Saratoga Springs offer watertaxis to DTD. AKL has two great restaurants in Boma's and Jiko
IF you were just going to use the hotel facilities I would go for either Y+B (if pool was #1 priority) or AKL if the animal savannahs tips the balance. For a longer trip the access to restaurants is a plus for Y+B, but for one night you'll be more than happy with what AKL has to offer.
Room choices, at AKL I would pay the extra to ensure a savannah view, maybe even check into the price of consierge, if you think you'd use the little extras one gets. If you decide to go for the Y+B and REALLY wanted to spoil yourselves you could look at getting a one bedroom DVC unit ( or rent DVC points from one of the DVC owners). If you decide to rent points MAKE SURE YOU GO ON A WEEKDAY. There is a large difference between Sunday to Thurday costs compared to Friday and Saturday nights. It could be as much as $400 difference for the night :O .
Don't forget if you book a hotel room for one night you can use the facilities on your check in day as well as your check out day. While you can't get your room until 3 or 4, there is no reason you can't pre-check into the resort and collect your resort IDS, call the front desk later to see if your room is ready. Again on checking out, OK you have to take your bags to your car by 11.00, but as I'm guessing most of your stuff you'll leave at the house, it'll only be an overnight bag you can throw in the car and when you've finished around the hotel pool you can drive home to shower off.
