Hi wormraper,
My stories are never short, so I apologize 
in advance. 
If you happen to come across any of my posts, you might notice that we have been sleeping on the Plank Luxe for about 4 years now. There is a long back story on how I ended up choosing it (and happy to share how we decided on a 1400 bed vs a 14k one), but it ultimately became our final choice, sight unseen.
For reference as it is import to notate this when speaking about mattresses, I am 6’ and around 220 lbs. I had actually grown to over 250 lbs. when we bought the PL. My wife is 5’4" and 135 lbs. Both start on our back 20% and end up on our side 80%, although I have been know to wake up on my stomach on some rare occasions.
From the first time after setting it up, and I laid on the Plank Luxe (PL), it was as supportive and comfortable as anything I had tested up to that point. It was definitely and AHHH moment. Some of the legacy brands felt great in the stores, and Aireloom felt fantastic, but their modern reputation for durability had been drifting south. Shifman was an easy yes for me even at $14K, and there really weren’t any downsides other than the price, but I was willing to go there, if the competitive side of me did not reveal itself. The ad’s for the Plank being touted as the firmest mattress on the market, simply made me want to try it.
Now that I test quite a mattresses on a somewhat regular basis, I have a Thermobalance Elite Firm, Titan Plus Elite, GlacierSleep Apex, another Brooklyn Bedding mattress in the guest room, and others in the test room. The PL is still the mattress we come home to. Quite frankly, it is more comfortable to us than any mattress we have slept on while traveling. Although, we did sleep on a Bear Elite Hybrid Firm and it was pretty darn comfortable.
Last year, at 65, I managed to hurt my back pretty badly from lifting mattresses and couches. Multiple herniated and compressed discs with spurs wrapped around the nerves. I couldn’t get out of bed for almost a week. That was really the only time the PL became too firm for me. Actually, firm is not the correct word. Not enough pressure relief to certain areas where my back needed some forgiveness. Thankfully, I ordered a half sized wedge Talalay topper from @EuropeanSleep European Sleep Works, and that little bit of additional pressure relief and slight incline was enough to make sleeping bearable while my back recovered. While I am at it, their Oxygen-Pillow and CubeLuxe pillow are fantastic. 
That experience actually gets into what I think you are wrestling with.
I didn’t initially choose the all-foam Plank, despite the claims that it is a step firmer. I’m pretty firmly in the camp that quality, robust springs will generally outlast foams, with some exceptions when you’re talking about good latex support components, although springs still have the edge when it comes to edge support. The edge support on the PL is also considerably better than the all-foam version, which mattered to me.
So, if you already know that you like the basic Plank feel, I don’t think going back to a Plank is a crazy choice at all. In your situation, I would probably look at the Plank Luxe as well rather than automatically buying another original Plank. You already know you can live with that general type of support, and with the two of you having different preferences, the Luxe gives you a little more cushioning without giving up that strong support underneath.
I will also say that the BB Thermobalance Elite gives its sister, the PL, a very good run, even for the extra money. As we get older, where the TBE really separates itself for us is pressure relief, particularly in the cover/panel. My wife constantly points this out when she goes into that guest room to read her Kindle and ends up falling asleep there.
I’ve actually come around to the idea that the TBE may be the better choice for us now as we get older. The support systems are extremely similar in how they support you. You sleep more “on” the mattress rather than “in” it, so you still get that cool, supported feeling, but there is just enough cushioning to take the sharpness off the shoulders, lower back and hips.There is a distinction there that matters. Everything between you and the floor matters, Sheets, topper, mattress protector, foundation, carpet, it all matters, Construction and organization of the layers, it matters. Craft mattress with 3 component layers, you get one feel, rotate the order of the layers, add mattress protector and things can change.
And I wouldn’t get too hung up on the 10/10 firmness ratings you are seeing. Those numbers need to be taken with a grain of salt, sometimes even within the same manufacturer’s lineup. Yes, the PL has a firmer and flatter surface. But the TBE doesn’t suffer at all in the support department. It just has a little more cushioning between you and the support system.
That is also why I wouldn’t necessarily assume that the Engineered Sleep Hybrid Classic Extra Firm being listed as a 10/10 means it is going to feel exactly like the firm side of the Plank. Construction, surface materials, quilting and how the layers interact can make two mattresses with the same numerical firmness rating feel very different. If you can test it, great. If not, I’d put more weight on the actual construction than the 10/10 number. Plus, the folks at @EngineeredSleep are straight shooters, you tell them what you are looking for, they will guide you to the best option they have available.
Latex is certainly worth considering too. Good latex can be extremely durable and supportive, and it has a very different feel from memory foam. But I wouldn’t spend $2,500+ on a complete latex mattress simply because someone told you latex lasts forever. You still have to like the feel. If you are already happy with the basic spring/support feel of the Plank, I don’t see a reason to abandon that simply in the name of durability.
And there are other mattresses I’d put on your radar. From my own experience, the DLX Premier and LatexLuxe have both given me a similarly excellent experience. We have also had the Titan Plus Elite and GlacierSleep Apex, both of which I thought were very good. With the Glacier Apex, I’d personally be looking at the firm version that is now available.
As far as your idea of doing two Twin XLs on a split king, I actually think that makes a lot of sense for you two because you have different comfort preferences. It gives each of you the ability to choose the firmness/feel that works for your own body rather than trying to find the one compromise mattress that works equally well for both of you. The question I’d spend some time on is whether you really want to be dealing with flipping/changing sides over the years or whether you’d rather have a construction that gives you the feel you want without relying on break-in.
If you are looking at an adjustable base, twin xl’s may be ideal. If you are looking at split feel as a couple. While buying direct may not be available for the TBE, buying from an authorized dealer like @yawnder, where they have the ability to order a split internal king, so firm on one side and medium or soft on the other.
And yes, I’d absolutely take advantage of the fact that you’re in Tucson and go lie on some different constructions if you have good independent mattress stores within reasonable driving distance. You don’t necessarily need to find the exact mattress you’re going to buy. You are really trying to answer some basic questions first: Do you still prefer that very firm, sleeping-on-top-of-the-mattress feel? How much cushioning can you add before your back says “nope”? And does a good latex or hybrid construction give you something you like better than the PL?
The biggest thing I’d say is don’t get too caught up in finding the “best” mattress. There isn’t one. There are a number of mattresses that can work extremely well for a particular person or couple.
At TMU, we understand the materials, springs, foams, densities, construction and all the other details, but we also understand that everybody has a budget. The trick is putting all of that together with the person’s body type, sleeping position, preferences and tolerance for pressure relief.
From what you’ve described, I don’t think you are starting from scratch. You already know that a very supportive mattress works for you. Now you’re really trying to figure out how much pressure relief you can add without giving up the support that keeps your backs happy.
That’s where I’d concentrate my testing. If you have any questions just follow up.
Hope this helps,
Maverick
