I'm going to agree completely with
@mmmears post above. She captured it all!
Although given your date (9/7) and the fact that 1) it's still very hot then, and 2) F&W will (probably) be going on, I'd probably lean towards Poly CL. The tropical vibe, the pools, and walk-ability to the TTC to get to Epcot give it a slight advantage in my mind. You'll probably also overlap with a
MNSSHP party or two, and the view of that fireworks show from Poly is great if you happen to be around the resort. (ETA: Not that the fireworks view from GF is bad, it’s great too, I just like Poly a bit better)
Not sure what kind of room/view category you are considering between the two, but I suspect Poly CL will be a bit cheaper than RPC (if that factors into the equation anywhere in your thinking).
Ultimately I could do either and be totally content in the decision.
Don't hesitate to ask if you have any questions. Happy planning!