How much bulk is afforded by 252 EVs in HP and 126 EVs in both defenses vs 252 EVs in both defenses with 4 EVs in HP?

Question

To give a pokémon as much bulk as possible to reduce the most damage on both sides, which EV spread is better for them? Is it better to give a pokémon 252 EVs to HP and then 126 EVs to both physical and special defense each, or is it better to give a pokémon 252 EVs to both physical and special defense each and then give them 4 EVs to HP?

Comments

You're looking for 128, not 126.

Answer 1

The answer depends on the Pokemon's base stats (specifically, the ratio between its HP stat and the defensive stat in question). In the vast majority of cases, 252/128 is superior to 4/252. However, this is not to say that you should use either of those spreads in practice.

You can easily prove optimal EV spreads using a damage calculator. Below are calcs showing that for a Pokemon whose base HP is equal to its defensive base stat, 252/128 is better...

252 Atk Salamence Dragon Claw vs. 252 HP / 128 Def Arceus: 103-123 (23.1 - 27.7%) -- 73.7% chance to 4HKO
252 Atk Salamence Dragon Claw vs. 4 HP / 252 Def Arceus: 94-112 (24.6 - 29.3%) -- 99.9% chance to 4HKO

...and when HP is much higher than the defensive stat in question, there is an opposite result:

252 Atk Salamence Dragon Claw vs. 252 HP / 128 Def Blissey: 360-424 (50.4 - 59.3%) -- guaranteed 2HKO
252 Atk Salamence Dragon Claw vs. 4 HP / 252 Def Blissey: 267-315 (40.9 - 48.3%) -- guaranteed 3HKO

However, if your takeaway from this is "always use 252 Def / 252 SpD on Blissey", then you are thinking about EVs the wrong way. Balancing EVs equally in both defensive stats is a lazy strategy that ignores realities about how you will use the Pokemon in the context of a team. In singles, Blissey almost always uses 252 HP / 252 Def or 252 HP / 252 SpD because a team of six Pokemon invites you to specialise your Pokemon. Your Pokemon don't need to be jacks of all trades (and masters of none) when you have five other team slots.

Comments

I see. Thank you for your response.
If this means much, I personally prefer investing the maximum amount of EVs into HP to at least grant some added survivability on both physical and special defensive fronts and then invest the maximum amount of EVs into whichever defensive stat I feel is going to help the individual pokémon survive more depending on if I am running a physical or special attacker and their defensive stats are close enough to each other (or sometimes just based on whichever is their weakest defensive stat between physical or special if there is a big enough gap).
Perhaps I am still not getting the right sort of takeaway from all of this which I apologize for, low tech player that I am.

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99% of the time, 252 HP (or 248 HP) is the optimal choice for defensive sets, both for the reasons outlined in the answer and the reasons you describe. So you don't need to worry about it much.
The only time 252 Def / 252 SpD is preferable is when you have a Pokemon with high HP, low defenses, and genuine reasons to invest equally in both stats. The main example that comes to mind is Wobbuffet in Ubers back before Xerneas made it run max SpD.