I don't think the "more stringent requirement" clause is there to help make decisions, but to cover the engineer and the owner in case of conflicts. The contractor can't determine the most stringent design, but the contract says they have to cover the price of it.
If the contract didn't have that clause, the contractors would say they bid the lighter, smaller whatever design in case of conflicts and would be due a change order when the heavier design was necessary. It's not fair, but the owners write the contracts, not the contractors.