Both are determining factors. But you're right, the more a mob attacks the more likely it is that central limit theorem applies. However, if you can get your hp total greater than the damage a mob can do in x rounds, it's going to be the superior choice. Imagine an extreme example of +10,000 ac or +10,000 hp. With complete heal, the HP will guarantee your tank doesn't die, but the ac allows for the possibility.
If you can't live through a max round, AC may/will be the better choice, depending on the mobs mean DI, number of hits per round, etc. There's an equation we could plug in to determine the probability exactly. There's 2 intersecting lines where one or the other is the superior choice. So it's definitely not a certainty either way (unless you can live through a max round with more hp, but couldn't with more ac)
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