Having both slept on firm mattresses and really loving them, sometimes it really comes down to the overall construction, balance, and thoughtful layering of the internal components. We actually have a 3" firm latex topper on ours now and are still very comfortable. I’m tempted to try a luxury firm topper to see how it would change the feel of our mattress, like the Helix Premium Topper with microcoils. Helix even makes a dual-firm mattress topper.
My mother-in-law’s mattress was a dual-firm model that I ordered for her—a DLX with a firm/medium configuration, and honestly, both sides feel great. I guess as we get older, a touch softer than very firm can have some advantages.
If you can find a showroom that carries a brand with four or five firmness levels, that might be the best way to make an initial comparison. The key is not to ask about firmness at all, just get right on and evaluate without any preconceived ideas. You’ll quickly eliminate what’s too soft (especially knowing it will only get softer over time) and narrow in on what truly feels good without bias toward firmness labels.