We just returned from an 11 day trip at GF and it was like Heaven!!! The Poly is beautiful as well. It just depends on the type of atmosphere you like and where you feel comfortable. Personally , I think you will have a great time at either place since they are both on the monorail and have easy transportation to anywhere. The pools at the GF are gorgeus, open and spacious. One has a slide and a graduated bottom. The poly has a volcano pool and it too has a slide.
The GF is victorian themed with pale colors. The Poly is darker, and themed (obviously) after the Pacific Islands.
Good luck!