I'm sympathetic to the first sentence I've quoted, but not so much the bit after the snip. You're not presenting an alternative to doing the maths; you're just pointing out more maths that is relevant!the sword and shield person will always, always, take less damage meaning they are less likely to be taken out of the fight or become combat ineffective at any point.
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Basically this entire thread has been about doing the straight up math without consideration to any actual in game practical situations. I would consider these other factors like how often you are hit and how much damage you take along with how often you are dropped to zero and how many spells have to be burned to keep each individual warrior in the fight.
After all, average number of rounds vs a given enemy without dropping to zero is just another calculation, that depends on the PC's AC (higher for sword-and-shield) and hit points and the enemy's chance to hit and average damage.