We have been in October before. It is still warm, but not the miserable hot that it gets in the summer...it is still warm enough to swim, and the food and wine festival is going on. Epcot is very busy on the weekends, but the crowds in the other parks have never been too bad.
We love Carribean Beach. The kids love the pool, and the sandy beaches are fun - the kids meals come with a sand bucket and shovel. You can lay in a hammock on the beach and see the illiminations fireworks right outside your room.
We feel that the extra for a prefered room location is worth it if you will be eating there much....