Depends on how much time you spend at the hotel. If you are pool people, than Beach Club will give you a better experience, even though its less time, it has Disney's best pool. It is also much more convenient since you can walk to Studios and Epcot, or take the boat, you can even Monorail to Magic Kingdom if you walk through Epcot, or just take busses. The Boardwalk area around the Beach Club is a great place to hang out and relax, and has some of the best restaurants on property (Shulas, Trattoria al Forno, etc). If the hotel is for sleeping only, take the extra 2 days. Personally I'd probably split the hotels to do Beach Club as long as we could and then toss in another hotel to hit the 10 days, Disney makes hotel swapping very easy, they'll transfer your luggage for you so all you have to do is check in again.
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