Having stayed at both resorts several times, which one you prefer is mostly a matter of personal preference. Each of the resorts has their own pros and cons, but typically its just whichever theme appeals to you the most. We personally prefer the GF, but weve always enjoyed our Polynesian stays as well.
Here are some of our personal pros and cons for each resort:
Grand Floridian:
Pros:
--MK is the first stop on the monorail after the GF
--Resort has a very pampering, luxurious feel
--Resort has tons of kids activities like the ladybug release, adventures in cooking, pirate cruise, princess tea party, etc.
--The vanity area is outside the restroom
Cons:
--At the end of a day at the MK, the GF is the last stop on the monorail
--Cost (the GF is more than the Poly)
Polynesian:
Pros:
--Wonderful restaurants (we love Kona Café and have enjoyed our meals at Ohana as well)
--Good location (you can walk to the TTC to catch the Epcot monorail if you dont want to have to transfer monorails)
--Relaxing, tropical theme
Cons:
--To us, the room décor used to be the main con, but now that the rooms are redone and look so nice online, Im hoping that wont be true anymore
--Tends to be a very busy resort, so some things might be more crowded than at the GF (like the volcano pool area)
--Sink is inside the bathroom, and there is only 1 sink in most rooms