Thanks, guys. I understand the trade-offs between comfort, handling, and tire life. My goal in this case is to get the best tire life (i.e., the tire wears evenly from center to edge), as the other factors are fine whereever they fall (this is a passenger car that doesn't get cornered hard).

However, I now have two contradicting answers to my main question.

One answer said that the same volume of air, supporting the same weight, will need to be the same pressure. Which makes sense. But then, there is also the answer that says the sidewall stiffness is a factor, and that's the thing that I'd expect to vary from tire to tire. And that also makes sense to me. So I'm still confused.