My son (aged 6) liked both, but preffered GF as it was much quieter. Only 6 children with 3 helpers when we went. He liked the Poly too as he made lots of friends....but he does like attention from the 'teachers', as he calls them!
Even though we stayed at the GF we put the kids in the Neverland Club at the Poly the night DH and I went out to dinner. The kids had done all the children's activities and GF and really enjoyed them, but the thought of going to Wendy's bedroom at the Neverland Club just appealed to them more.
BTW they had a great time - so much so that they were disappointed when we went to pick them up!