Have you ever stayed at the Grand Floridian? If not, why not give it a try? We love trying different resorts at Disney. You know you won't be staying at a dump, so the worst that can happen is you decide GF is not your speed and choose not to stay there again.
I presume you're asking about a Poly studio and not a Poly bungalow, correct? The studio is about 447 square feet compared to a standard room at GF which is about 440 square feet (unless you're staying DVC at GF?), so the difference in room size is negligible. Assuming you're staying in a standard room at GF and not DVC, then you would not have the kitchenette that you'd have in a Poly studio. However, a Poly studio would only have one real bed with a pull out sofa and a pull down bed. That may make the room feel crowded. At GF, you'd at least have two real beds plus the daybed, which I think would be more comfortable.
If it were me, I'd give GF a try. As PP said, you can't really go wrong with either.