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You know, in my opinion, the choice isn't that obvious.
If in an example where a hero took two defenses (becoming 2-12), they are attacked by a hero with parameters 11-3, then these heroes will deal damage according to one formula, and the damage will be lower. But if the hero on the arena had initially taken attack, it would be 4-10 against 11-3, and that's a completely different game! The choice must be made based on the situation. Maybe we have
tactics + mass haste. Then go for attack.
1. It's very difficult even by week 2 to estimate the ratio of attack and defense of the opponent (perhaps with the exception of Barbarians and Swamp), because stat artifacts are very powerful. This isn't HoMM5, where nothing more than +2 to everything simply exists. And if the racial profile is attack, then attack will be the highest.
2. The fight with the enemy is essentially one single battle, while everything else is against neutrals with similar attack and defense values. That's who you should orient yourself toward.
3. You often don't know who has the right of first move or if tactics will work at all.
Of course, if you are sure that you strike first, then attack bonuses increase, but not as much as it seems. After all, each subsequent point of attack gives a smaller relative increase than the previous one. While for defense, it's the opposite—a larger one.
And since we're talking about a specific example—the choice on the arena with parameters 2-10—then your example regarding the battle with an enemy hero is statistically not very correct. Based on the ratio of stats and their sum, I would logically assume that the game is being played by Swamp, end of week 1 or beginning of week 2, if a skilled player is playing. The stage of taking various crypts is over (and if they are encountered, they pose no problem), the main thoughts are how to break into a hive or a fatter "can" [strong neutral camp], the turn hasn't even reached the enemy yet. Therefore, as the key battle of this game stage, I would consider taking "can" 3 or 4 (if Tazik has offense or something good in his book). The main damage dealer will be a stack of 2 hydras, but they must be preserved, and then some math. We already have hydras with parameters 16(18)-18(28), and the question is where to distribute another 2 points if fighting griffins 8-8. If +2 attack was taken, then hydra damage will increase by 1.6/1.5 = 1.0(6) times. If +2 defense is taken, then the damage received will drop by 0.45/0.5 = 0.9 times. That is, either +6.7% damage to griffins, or -10% from griffins. All other stacks in the battle are essentially meat, and for them, defense is definitely more important. A few choices in favor of defense can quite possibly move a hero from "normally takes 3" to "normally takes 4" on day 8-9.
Yes, there are cases where you're forced to orient toward attack—shooters and wolves—but that might come back to haunt you later. After all, in HoMM3, the role of shooters is not very great even by week 3.
Of course I understand that in Dragon Utopia, for example, the choice is made in favor of defense, but in crypts, "cans", or external dwellings, at early stages of the game, I would still give preference to attack.
I'll give my general IMHO: investing in the attack parameter often only allows you to pass already passable battles more easily (the exception being mostly finishing off with shooters at the limit of the stack so they don't block later), while investing in defense allows you to pass a battle that you shouldn't attempt without it—either a wipe or key losses.