Given that you sleep in different positions, you may both be happiest if you look for a mattress where you can customize the firmness on each side as the back sleeper may want a firmer mattress, where you may want a little more cushioning, as mentioned by Arizona Premium. Latex mattresses are often customizable in this way, although we offer innerspring and hybrids that are also customizable and very breathable. Here’s a link to an article we wrote for Mattress Underground about hybrid mattresses, if you are wondering about those.
If you are looking to sleep cooler, natural materials are typically more breathable than anything synthetic - this goes for your bedding and your mattress.
You mentioned that it feels like there is a dip. When you are trying out a mattress, have someone look at your spine as you sleep on your side. Your spine should be neutral/straight - this is a sign you are getting the proper support and should help with the pain issue.
If we can help in any other way, let us know! We love to answer questions.