Artisan choice - comparison with Nights in iron?

Hi there - we would like to buy a new mattress to replace this one - that's now over 10 years old
I am 13.5 stone and my wife is circa 10 stone - so we would suit a Med tension....but deliberating on what Artisan model to select - as we are uncertain how "soft" is soft eg in the Luxury model.....any view on which might be comparable to the linked mattress above?
my wife also suffers from lower back pain - not sure if this is due to carrying 3rd son, or the current mattress...or a combination of both (the issue is muscular not spine or skeletal).
Any thoughts...we are deliberating between the Artisan Naturals, the luxury, and the Bespoke 004. Would the 004 feel the same as the Naturals or as half way house between the Natural - Luxury?
Thank you
steve