Hp vs Def IVs

If one of the natural stats is extremely lower, you would prefer higher IV on that stat, for example, you will definitely prefer a 15Def 0HP Chansey than 0Def 15HP Chansey. Or you will prefer a 0Def 15HP Cloyster than 15Def 0HP Cloyster.

In the case their natural stat for Def and HP are similar, in this game, I would prefer HP. Since HP can be turned into energy, and you will have 2X HP when defending.

To your specific Chamander case, I would prefer the one with more HP, but the difference in real world is negligible.

Defence. If the extra defence makes the difference between 1 point of damage per quick attack, it results in way more EHP than an extra hit point, which the most that you'll get from 1 point in stamina.

HP being turned into energy is irrelevant, it's one extra point of energy as a defender when you're at 1 HP, so your Pokemon is going to faint the next attack anyway, where that 1 extra point of energy has done nothing.

If that one point in defence doesn't make any difference to damage reduction, neither would 1 extra HP anyway. Basically, the potential gains from 1 defence is higher than the potential gains from 1 extra point in stamina.

The second one, per the result of square rooting.