I agree with BCV.....your party of 5 means you need a bigger room with a sofa sleeper arrangement. POLY has it. WL does not. We stayed in FIJI last spring...loved it. My kids would not leave the volcano pool. My wife and I actually like the WL much more, but we are a party of 4. If the cost is not an issue, go for the Poly and do some research here on a building request. It is a big resort, but very manageable. Being "on the monrail loop" is a plus for fast access to the MK. But don't miss the boat ride (resort launch) either.
They are going to love it.
Artemus