It also makes sense to compare the relative impact each card has on the situation you are considering if you are assuming they are both present. In the case of a board where a engine enabled by Herald is present, you could replace Ironmonger with Pearl Diver and build essentially the same deck. It is true you want to open with Ironmongers over Herald in this case, but that fact does not support the conclusion that Ironmonger is a better card, or more important in this scenario.
Let me be clear that I am not taking a side here. Ironmonger is a good card almost always, but a key card only when it is the only Village and can reliably be used as such. Herald is worse-than-Pearl-Diver-bad on some boards, and arguably better than Lost City on others. Which one is the 'better' card? Man, 'better' is a subjective thing that can be defined in a multitude of ways. The way you define it will determine which of these cards fits that definition more closely. (Surprise! You aren't arguing about which card is better, you are arguing about what 'better' means. Good luck with that.)