We do like to obsess about weight, despite frame builders telling us again and again that it isn’t high on their list of criteria. I have three very different steel touring bikes, the Mercury being the lightest and a Surly Ogre the heaviest. The difference in frame weights will be less than half a kg, though the difference in bike weights is likely to be around 4kg. If a bike is designed well for it’s intended purpose it’ll weigh whatever it weighs, that’s not to say they’re all the same, just that it isn’t necessarily reflected in the weight. You can add a lot of strength and stiffness, and consequently ride feel, without a proportional weight increase. Take the brochure for the 700c Nomad, it says that frame with a 853 fork is the same weight as a Mercury frame with a ST fork.
Only Thorn will know why they choose the steel they do for their different models, and they’re unlikely to tell us! Cost, availability, specification, may all play a part, as will marketing. I do know that heat treated Cro-Mo is available from several suppliers, and is exactly (Or close enough to be indistinguishable) the same as Reynolds 725. Maybe there is no readily available equivalent to Reynolds 853, or maybe Thorn decided those spending that sort of money will want the badge… we don’t know.
Back to the question! If I was sure I wouldn’t be carrying camping weights, I wouldn’t consider a Nomad, or the Shand Tam. It would be the Mercury from Thorn and refine it with build and wheel size, or I’m not sure what from Shand (Maybe the Stoater, but the range seems to have changed since I was comparing)
I like my Mercury, 700c and 853 fork, mine is the largest size and I’m no lightweight, I’ll take my lightest camping gear if I’m going somewhere than needs it, but I wouldn’t choose to use it for a camping trip.
In the end most bikes will do most things, but to get the ideal one requires a good idea of it’s use. If you’re not sure, maybe do some touring on whatever you can before making a big investment.