And one could easily justify having both… but if only one watch and that one is a Nautilus then perhaps your steel 5726 would be the one. Steel being a far more wearable metal for frequent wear and the 5726 is such a versatile reference. Can play both formal and playful roles.
The 5740 is a top elegant Nautilus. Very dressy with a sporty element. I think the fact it is made in a precious metal makes sense and would not want to see it in steel personally (even though I would love to own one if it were to exist). I think it is a strong contender for an only Patek watch of one was to have one. But not an only watch contender unless one at least is not allowed a g-shock as well…
For a Nautilus pair I would vote 5740 and a 5980/1A… steel sport chrono and a grand complication… but that is just my 10c worth of rambling