Hi jacks,
The first thing is that you are certainly considering two items using good quality and durable materials – in effect choosing between “good and good”.
Looking back at you previous “checklist”, both of these mattresses would be quite breathable, they are both available in firm, they are domestic products, the latex is “well-certified”, they can both be used on an adjustable bed frame, and Nest has a good return/exchange program, and they both fit within your budget. So that’s all good news.
The Nest Bedding Q3 comes as a component-style system and can be had as a firm/medium option. It uses Dunlop latex that can be has blended or organic certified. There is a 6" piece of firm Dunlop and a 3" piece of soft Dunlop. For the “firm” comfort, you’d arrange the product with the 6" firm layer over the 3" soft layer. For the “medium” comfort, you’d reverse the layering order.
The Nest Bedding Hybrid Latex can be ordered as a Medium or as a Firm version. This mattress is an innerspring style mattress using an 8" Quantum Edge pocketed spring unit for both versions. The Firm version uses a 3" piece of firm Dunlop latex on top of the spring unit, and the Medium version uses a 3" piece of medium Dunlop on top of the spring unit.
Some people prefer the feel of an innerspring product with less comfort padding material, where others prefer an all-foam product. There really is no right answer, only what works best for you (which unfortunately I can’t predict 
). There is some more detailed information regarding the difference between a spring and latex support core in this post that you may find interesting.
As you’re aware, Nest Bedding is a member of this site which means that I think very highly of them and that I believe that they compete well with the best in the industry in terms of their quality, value, service, knowledge, and transparency. And they do offer a good exchange/return policy should tings unfortunately not turn out as well as you might have hoped.
Phoenix