We struggled with a similar problem and settled on a wool topper over latex. Wool is superior to cotton in terms of moisture and temperature regulation, and it is resistant to dust mites and bacteria. The wool topper has a cotton cover, so there's no direct contact with the wool (plus there's the mattress pad and bottom sheet on top of it).

It's the best of both worlds--the support and resilience of latex, with none of the temperature or moisture problems. The best way to describe it is that it feels like a brand new futon mattress (before they get compressed and rock hard) every time you lie down on it. Hands down the most comfortable bed I've ever slept on.

Edit: Almost forgot, you can dial in the support in this setup by starting with a relatively thin, extra firm latex mattress and wool topper. If that is too hard, add a softer latex topper between the wool and the mattress.