Everyone who said your utility function needs to track the actual problem domain is correct. Here’s my advice on how, specifically, to do that.
The utility of weapons and armor in a D&D-like RPG is that they help you win battles. How useful at the margin better armor or a better weapon are is going to depend on what you’re fighting: more damage is useless if you already are killing a goblin with every hit, for example. So you need to come up with a reasonable list of encounters and a way to estimate how good a kit is against each encounter. This probably means calculating how likely you are to hit that opponent, how likely it is to hit you, and how much of the other’s health each of you will knock off with each hit. If you hit 80% of the time for 20 points of damage and it has 50 health, you’d expect, on average, to need 2.5 hits to kill it and 3.125 rounds to get those 2.5 hits.
The ratio of expected-rounds-for-you-to-kill-it to expected-rounds-for-it-to-kill-you is probably a good heuristic to use in a combat system derived from D&D, and you can probably figure it out with some basic statistics, but it can fail in some situations: it really doesn’t help you to do 90% damage instead of 51% if you’ll still need two hits to kill it. A more complex alternative would be to calculate instead how likely you are to win the fight. First calculate how likely the opponent is to die in one hit, two, three and so on; then how likely you are to have one hit on the first round, one hit on the second, two hits on the second, and so on, up to a reasonable number of rounds. Repeat for its attacks on you, and you can calculate the odds that each of you will die on each round.
For a much more complicated combat system, where generating a probability tree would be infeasible, you might need to do some Monte Carlo simulations of battles to get an accurate optimization.
The simple heuristic you want, though, is probably something like, (YourChanceToHit * YourDPS * YourHealth) / (TheirChanceToHit * TheirDPS * TheirHealth). That captures the intuition that the following are roughly equivalent: hitting twice as often, needing half as many hits to kill them, getting hit half as often, or surviving twice as many hits.