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.