Is It Better to Run Max HP or Distibute EVs in Defences?

Question

I was curious on whether running HP or mixed defences was optimal for getting the enduring hits on a bulkier, high damage Pokemon. Let's use this hypothetical set-up:

Reuniclus@ Life Orb
Ability: Magic Guard
Quiet Nature
EVs: 252 SpA/252 HP OR 252 SpA/~150 Def/~100SpD
-Psyshock
-Focus Blast
-Shadow Ball
-Trick Room

Which one is bulkier? My brother runs these types of setups all of the time, and I want to know if he does it right or not.

Answer 1

Usually, running more HP investment rather than mixed defense investment is better. With more HP you're able to take more hits which is why Pokemon like Chansey are commonly used. Even though it has poor defenses, it can still take sponge attacks thanks to its huge HP stat. Even if you do run mixed defenses with that Reuniclus set, the difference is very minimal.
252+ Atk Huge Power Azumarill Waterfall vs. 252 HP / 0 Def Reuniclus: 202-238 (47.6 - 56.1%) -- 82.4% chance to 2HKO
252+ Atk Huge Power Azumarill Waterfall vs. 0 HP / 150 Def Reuniclus: 169-199 (46.8 - 55.1%) -- 69.5% chance to 2HKO

Answer 2

For that specific set it's more suited to a Special Tank so I would put the IVs in Sp. Defence. But it all bases on the type of Pokemon you want to run, for a Wall Pokemon put IVs in HP but for a tank put them in either Defence or Sp. Defence