Hi craiggroves91,
Welcome to the Mattress Forum! 
So, you’re a bit of a “rotisserie chicken” - rolling around in different positions throughout the night! :lol: But sleeping mostly on your stomach. Sleeping on your side usually requires the most surface conformation, as you mentioned the difference between the width of the shoulder and the waist/hips. Sleeping on your stomach usually requires the firmest surface comfort, and you don’t want too much conformation that would accentuate the lordotic curvature of the lumbar region. Some people sleep totally prone (needing the firmest surface comfort) while others bring their knee up a bit and sleep with a “1/4 turn” when sleeping on their stomach (can deal with a bit more surface conformation). The nice thing with the Best Mattress Ever (BME) is that the top two layers of the product are highly point elastic, so even a “firmer” version will still contour well to your body.
Your best step would be a phone conversation with Brooklyn Bedding, as opposed to an email or chat. Brooklyn Bedding is a member here of this site, which means that I think highly of them and their product knowledge. Their detailed knowledge of their mattresses and how they fit with different body types and sleeping positions, plus your own personal feedback on what you’re currently using and what works best for you personally as reference points, will combine to give them better information to make a suggestion between either their “Medium” or “Firm” model. Additionally, you can then inquire about their 120 night trial policy should your purchase unfortunately not turn out as well as you had expected.
While the latex used in the BME is very breathable, all of the materials, layers, and components of a sleeping system that are closer to your skin will have a bigger effect on airflow, moisture transport, and temperature regulation than materials, layers, and components that are further away from your skin, and softer mattresses or foam toppers will tend to be more “insulating” and for some people can sleep warmer than firmer versions of the same material. If you’re interested in exploring this further, there is more about the many variables that can affect the sleeping temperature of a mattress or sleeping system in post #2 here that can help you choose the types of materials and components that are most likely to keep you in a comfortable temperature range.
I’m interested to know what you decide upon and any advice you’re provided by Brooklyn Bedding in that process.
Phoenix