I think the GF views are better because you are closer to the castle. However, the views from the Polynesian are breathtaking as well because you can see the entire MK area. You see the lagoon, the GF, the Contemporary, the castle, Space Mountain.....it's a wider view. The GF has some rooms with views of the lagoon, castle and Cont., and I think that's from Big Pine and Boca Chica buildings. I'm not 100% sure on that, though. The views from Sago Cay and Conch Key are just going to be of the castle, part of the lagoon and a glimpse of the Cont. We stayed in Sago Cay for our honeymoon and I loved the view. I guess it all depends on how close you want to be to the castle.
I'm not sure what the weather is like in April. I want to say it's more pleasant than the summer months, which can be brutal. I've never been in April.....sorry.