I prefer both the AKL and WL over OKW, but to go for an extra week meals included for the same amount? No contest. IF I could afford to be off work that long, I'd do OKW. I'd also probably consider renting a car for at least part of the trip (there are places to rent cars on property, and they'll pick you up at your resort and bring you to the rental centers to pick up your car). This way, I can more easily visit the other resorts.
I'd also add, if you can get the same 3-week deal staying at Saratoga Springs (and you indicated that it would be), I'd probably choose that over OKW. Keeping with lovedonald's comments, Saratoga Springs is MUCH closer to Downtown Disney then Old Key West is, in fact the Congress Park section is less than a 10-minute walk to the bus stop at the Marketplace (while also being pretty close to the main building), giving you much easier options to get to all the other resorts, even without a rental car. The disadvantage is that the studios in OKW are bigger than the rooms at SSR, so if you'll need more space (I've no idea how many people are in your party, any more than 2 adults you may prefer the extra space at OKW).
Aaaaand to throw one more option out there to make your awesome choice that much harder, if it's that much cheaper to stay in OKW or SSR, have you thought about 2 weeks in one of those resorts, but in a larger room? Is that even an option for you with this promotion? I know the full kitchen won't be used by you with your free dining option (except the fridge may come in handy for leftovers or uneaten snacks), but the separate bedroom might be nice for such a large stay, especially if you are a party of more than 2 people. Plus, you'll get in-room laundry, and a whirlpool tub.