After staying at the Polynesian with my two DSs -- 4.5 and 7... my DW and I pretty much decided that we would probably never stay anywhere else.
Mine are just a bit too young to really get into the theming and the ambience of the islands, but I think they enjoyed the overall atmosphere of the resort. DW and I loved the convenience and all other aspects of the Poly.
Your daughters are older... I can see them liking the Polynesian as well... but they might well be of an age where the Grand Floridian would appeal. Polynesian is very informal and kid-friendly; from what I've seen, the GF is much more formal and perhaps not as appealing to younger children.
Just my $00.02...