Best option after attuning to Shield of Missile Attraction

Question

Let's say the party tank has picked up, and attuned to, a Shield of Missile Attraction, which states the following:

While holding this shield, you have resistance to damage from ranged weapon attacks.

Curse. This shield is cursed. Attuning to it curses you until you are targeted by the remove curse spell or similar magic. Removing the shield fails to end the curse on you. Whenever a ranged weapon attack is made against a target within 10 feet of you, the curse causes you to become the target instead.

It's noted that the "curse" portion of the shield does not require that the character carry the shield, and also does not force the character to carry it (such as some cursed weapons do). Consideration was given to having the curse removed, but it is decided to not do so, as being able to completely protect other, more squishy, party members from ranged weapon attacks, by simply standing nearby, was considered a net benefit.

The downside, however, is that a character is only resistant to ranged weapon attacks while holding the shield.

So the question is, where does the break point exist that the character should hold the shield (and gain the resistance to ranged weapons attacks) versus the alternative of holding their +2 shield instead (which doesn't require attunement)?

As a limitation to the question, limit the comparison against the ranged attacks that the Resistance is effective against.

Answer 1

Attunement favors the +N shield

It's noted that the "curse" portion of the shield does not require that the character carry the shield, and also does not force the character to carry it (such as some cursed weapons do).

Not only do you not need to carry it, you don't even need to stay attuned to it. The options for ending attunement are as follows:

A creature’s attunement to an item ends if the creature no longer satisfies the Prerequisites for attunement, if the item has been more than 100 feet away for at least 24 hours, if the creature dies, or if another creature attunes to the item. A creature can also voluntarily end attunement by spending another Short Rest focused on the item, unless the item is Cursed.

The prohibition on ending attunement to a cursed item only applies to the last clause. Therefore, you could still end attunement by removing the shield of missile attraction from yourself by a distance of at least 100 feet for 24 hours, or by having someone else attune to it. (I suppose you could also die and get revivfied, though this is rather extreme and costly for anybody who's not a Zealot barbarian.) See also this question.

Since a +N shield does not require attunement, you could end attunement to the shield of missile attraction, keep the curse, and use the +N shield with zero attunement slots occupied by shields.

On the other hand, if you wanted to keep the damage reduction from the shield of missile attraction, the attunement rules state:

Without becoming attuned to an item that requires attunement, a creature gains only its nonmagical benefits, unless its description states otherwise. For example, a magic Shield that requires attunement provides the benefits of a normal Shield to a creature not attuned to it, but none of its magical Properties.

and therefore you would need to maintain attunement with the shield of missile attraction to keep the damage reduction. Thus, on top of competing with the +N shield, the damage reduction is also competing with any other magic item that you could use the attunement slot for.

The damage resistance is less valuable for barbarians

Speaking of barbarians:

• Barbarians have resistance against bludgeoning, piercing, and slashing damage while raging.
• Damage from ranged weapon attacks is likely to be bludgeoning, piercing, or slashing, since all standard weapons deal damage of these types.
• Damage resistance doesn't stack (PHB p. 197).

Therefore, if you are a barbarian, you will benefit relatively less from the resistance to damage from ranged weapon attacks.

Answer 2

Damage resistance is almost always better than +2 to AC

The calculation for the benefit of damage resistance is pretty simple: it reduces incoming damage by slightly more than 50% (slightly more because of the round down rule). In order to match that benefit, an AC bonus would need to cut your chance of being hit by 50%. Generally, each +1 bonus to AC subtracts 5% from the enemy chance to hit¹. Hence, a +2 bonus to AC subtracts 10% chance to hit. If the enemy initially has a 20% chance to hit, subtracting 10% represents a 50% reduction in hit chance, which matches the target we set based on damage resistance. So, if the enemy only hits on a natural 17 or higher, then a +2 bonus to AC approximately matches the overall damage reduction of damage resistance. Note that such a low chance to be hit is extremely rare. For example, if the enemy has a +8 to hit, then your AC needs to be 25 (before adding the +2 bonus) to achive such a low chance to hit.

...Except even that isn't quite good enough. At these extremes, critical hits can't be ignored. If only a natural 17 or higher hits, then 25% of hits will be critical hits. Your +2 to AC doesn't help against these at all, but damage resistance does. This means that even when the enemy hits on a 17 or higher as described above, damage resistance is still significantly better. So in order for +2 to AC to beat damage resistance, you would need a situation where the enemy only hits on an 18 or higher. Even worse, the benefit of +2 to AC actually goes down if your AC increases further, because if the enemy needs a 19 to hit, then +2 to AC only subtracts 5% from their hit chance, and subtracts less than 50% from their total incoming damage. And if the enemy only hits on a natural 20, then +2 to AC does nothing at all. So actually, a +2 to bonus to AC is better than damage resistance only if the enemy's minimum roll to hit you is exactly 18, no more and no less. Even if you actually had the absurdly high AC needed to make this happen, different enemies have different attack bonuses, so few enemies will have the exact attack bonus that makes the AC bonus better. This means that even in the best case scenario for the +2 to AC, it still probably falls short of damage resistance most of the time, unless all your enemies have exactly the same attack bonus.

Which shield should your tank equip?

The conclusion is clear: against ranged weapon attacks, the shield that grants damage resistance is unequivocally better than the shield that gives +2 to AC. However, against any other kind of attack (including ranged spell attacks), the +2 to AC is obviously better, because the cursed shield does not grant any extra bonus against these attacks. So, your tank will need to make an educated guess as to what kind of attacks they will mostly be facing and equip the appropriate shield. Or alternatively, the party could alter their plans to engage at range to force the enemy to rely on ranged weapon attacks whenever possible, in order to maximize the benefit of the shield.

What about a +3 shield?

As you can see, with a +2 to AC, the math works out in favor of damage resistance in nearly all cases. A +3 to AC is slightly better, but it's still going to fall short of 50% damage reduction the vast majority of the time. I'm not going to go through all the math again, but I think the +3 to AC is only better if the enemy's minimum roll to hit is between 16 and 18 (again, it can't be lower or higher than this because of how critical hits work). This still requires an absurdly high (pre-bonus) AC and still has the same problem that not every enemy has the same attack bonus, so again, damage resistance comes out on top almost all of the time.

If you're wondering about a +1 to AC, it can never reduce your incoming damage by 50%, so it is always worse than damage resistance in all cases.

---

¹The exception to this is when the mismatch between the enemy's attack bonus and your AC is such that enemy already hits on a natural 2 or misses on a 19, which is probably not the case here.