Pretty different experiences in so many ways.
Neighbors will impact everything, like anywhere. I find Poly resort rooms relatively good with neighbor noise, but nowhere is going to be immune from really noisy folks.
Generally speaking FW Cabins will be the quieter experience overall, in my opinion. BUT just because there's physical distance between cabins and you're "in the woods" doesn't mean you're isolated from noise. The cabins are still close together and there is a pretty steady stream of vehicles, golf carts (some make more noise than others), and various WDW utility/housekeeping carts (mostly gas powered) that can create noise. Also people tend to hang out on their porches, cook out, etc. It's not unheard of for larger groups to get several cabins and "combine" forces - pop a tent, put up chairs, etc.
I will also add the cabins have what I consider a pretty annoying dehumidifier that can't be turned off - in the quiet of night the hum can be annoying for some. It didn't bother me in the bedroom but did bother me when sleeping on the pull down in the living room. YMMV here, some have reported not hearing anything, some have reported the same annoying hum.
Poly is a bustling resort but has remarkably quiet portions as well. Room location and neighbors are key, things you can't really control.
Poly is my favorite resort but I also adore FW. They're both gems. I would personally be making the decision on what type of WDW vacation/location I want over possible noise at either, IMHO.