Hi Britchap,
There are two things that I would take into consideration if I was in your position.
The first and most important would be which one is the best match for you (or whoever would be using the mattress the most) in terms of PPP (Posture and alignment, Pressure relief, and Personal preferences). In other words … which one do you (or the person that would be using the mattress the most) sleep best on.
The second would be which one is likely to be the most durable (although if you would only be using this on occasion or in a guest bedroom or for a little while then durability wouldn’t be as important an issue). It’s only possible to know this if you know the specifics of all the materials inside each mattress (see post #4 here). Since it’s not likely that you would be able to find out the information you would need to assess the durability of the mattresses this probably wouldn’t be a factor unless one mattress is clearly near the end of its useful life and is in poor condition.
One other factor that may make a difference is that if you have a mattress that is very firm and there are no soft spots or sagging in the mattress then you can add a topper for additional pressure relief and comfort (see post #2 here and the topper guidelines that it links to). The advantage of this is that the upper layers of a sleeping system are generally the weak link of a mattress and if you have a topper instead of using the same layer inside a mattress cover then it can be easily replaced without having to replace the entire mattress. The biggest risk with this topper/mattress combination is that choosing a topper of the type, thickness, and softness level that works well for a specific person and mattress where you haven’t tested the combination in person can be almost as difficult as choosing a mattress.
I would keep in mind that firmness and support aren’t the same thing and that a mattress that is too firm and a mattress that is too soft can both provide poor support. Sleeping on a wooden floor for example would offer poor support under more recessed parts of your body (such as the lumbar curve on your back or your waist on your side) and this part of your body would end up sagging with little support underneath it. Sleeping on a mattress that has comfort layers that are too thick or soft may be more supportive under the recessed curves which would be filled in with the softer comfort layers instead of “air” but would offer less support under the heavier and more protruding parts of the body such as the hips/pelvis. The goal of a mattress is to help you maintain good alignment in all your sleeping positions which means that it needs to have a combination of firmer support layers to “stop” the heavier parts of the body from sagging into the mattress too much (which I call primary support) and it also needs some softer comfort layers to “allow” the more protruding parts of your body (such as your hips and shoulders) to sink in enough that it fills in the gaps in your sleeping profile (which I call secondary support) and provides good pressure relief. There is more about primary support, secondary support, and pressure relief in post #4 here. There is also more about some of the other preferences that may be important to you and contribute to the quality of your sleep in post #4 here.
The bottom line for me would be that I would keep the mattress that is the best “match” in terms of PPP and that I slept best on.
Phoenix