Defence IV vs Stamina IV

I also have been wondering about this. I think you can figure this out by doing some (a lot of) math based on the HP calculations, defense stat calculations, and damage calculations (but I'm not entirely sure how since there are so many variables, and I don't intend on putting in that kind of effort).

My theory is that since IV only adds a maximum of 15 points to a pokemon's base stat in the calculation of the complete stat, a pokemon with a large base stat in either defense or HP will be less affected by its respective IV's. The take away being that you should go with the pokemon that has higher IV's in its weaker base stat. E.g. a 0/15/15 chansey would be better defensively than a 15/15/0.

Then again, I'm not sure how important defense is in the damage calculation. Does a higher defense stat significantly reduce the HP reduction each attack does, or does defense not weigh in so much that having an extra 5-10 HP points results in the attacker needing to launch a few additional attacks. Feel free to dispute this if you can do better guess-math than me.

more DEF IV = save potion

more IV that pokemon lack = better duel ability.

Many researchers give the same weight to stamina and defense in order to figure out the tankiness of a pokemon (often this formula is used def*sta). Generally speaking I prioritize the IV stat my pokemon is weak in. E.g Venusaur: 200 def, 160 sta I would prioritize stamina, since +15 IV in its base stat value would result in a higher % increase.

As Defender HP get's doubled. I'd go for a high Stamina IV for sure if it's gonna be a pokemon defending gyms.