This thread is in full silly mode. +2 dice is almost always better than +2 piercing, and +4 dice is better than +2 dice & +2 Piercing.
The exact math doesn't follow the rule of thumb perfectly, but generally, armor stops about .5 damage per point. Sometimes it stops as much as 1 damage per point (if your armor score is low compared to the number of dice. Other times it stops as little as 0 damage per point (if your armor score is high, compared to the number of dice.
You can see this by imagining a 3 die attack against a 6 armor creature. That 6th point of armor is largely worthless, and will only make a difference if the attack rolls all non-crit 2s.
Conversely, if you imagine a 3 die attack against a 1 armor creature, that 1 point of armor is very likely to be relevant. Any die that comes up non-crit (1 or 2) will make the armor useful.
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So what does this mean? Against a highly armored target you do not want piercing if the alternative is more dice. Piercing is less useful against highly armored targets, because Armor is less useful on highly armored targets. Taking an Iron golem from 5 arm to 4 arm is not helpful with a 3 die attack. 4 arm is still enough to soak all the non-crit damage most of the time, and whenever that happens, the piercing is worthless.
If that piercing had been +1 melee instead, the expected increase in damage value is at least .5 because of crits, and maybe higher if you can go over the top of the armor with normal damage.
Of course, against a very low armored target, you do not want piercing. Piercing is useless against unarmored targets, because armor is zero bounded. You can't have negative armor.
And in the Goldilocks zone, of neither too heavily armored nor too lightly armored, piercing is still almost-never better than just adding one more die.
The only time +X Piercing is better than +X attack is when it stretches across multiple attacks per activation, like when the creature both guards and attacks, or uses battle fury. But that's not because of the dice math, it's because of the limitation on melee+X in the rules. But then, you're comparing +X piercing to +0 dice in a corner case. And a printed 6 die melee attack is always better than a 4 die +2 Piercing attack, all other things being equal.
So, I'll say it again, if the alternative is +X dice, you never want +X piercing instead.