HP or Def + Sp.Def?

Question

Is it safe to assume that:
generally, it is better to invest more on a Pokemon's HP (through EV training) than on its Def and Sp. Def, given that the Pokemon's base HP, Def, and Sp. Def stat are equal? (Moreover, it is hard to predict whether an opponent will use a Physical-based or a Special-based attack)

Answer 1

252 HP calm (average damage is 36% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 252 HP / 0 Def Mew: 138-163 (34.1 - 40.3%) -- guaranteed 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 252 HP / 0+ SpD Mew: 127-150 (31.4 - 37.1%) -- 83.4% chance to 3HKO

252 SpD calm (average damage is 39% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 0 HP / 0 Def Mew: 138-163 (40.4 - 47.8%) -- guaranteed 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 0 HP / 252+ SpD Mew: 100-118 (29.3 - 34.6%) -- 7.3% chance to 3HKO

176 Def/76 SpD calm (average damage is 38% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 0 HP / 176 Def Mew: 117-138 (34.3 - 40.4%) -- guaranteed 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 0 HP / 76+ SpD Mew: 117-138 (34.3 - 40.4%) -- guaranteed 3HKO

So if you have to put 252 EVs in HP or defenses, then 252 HP is probably the best on average. But why are you limiting yourself to 252 EVs when the game gives you 508?

4 HP/252 Def/252 SpD calm (average damage is 33% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 4 HP / 252 Def Mew: 109-130 (31.8 - 38%) -- 94.5% chance to 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 4 HP / 252+ SpD Mew: 100-118 (29.2 - 34.5%) -- 5.5% chance to 3HKO

252 HP/4 Def/252 SpD calm (average damage is 32.1% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 252 HP / 4 Def Mew: 138-163 (34.1 - 40.3%) -- guaranteed 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 252 HP / 252+ SpD Mew: 100-118 (24.7 - 29.2%) -- 99.9% chance to 4HKO

252 HP/180 Def/76 SpD calm (average damage is 31.6% of full HP)
252 Atk Aegislash-Blade Iron Head vs. 252 HP / 180 Def Mew: 117-138 (28.9 - 34.1%) -- 2.1% chance to 3HKO
252 SpA Aegislash-Blade Flash Cannon vs. 252 HP / 76+ SpD Mew: 117-138 (28.9 - 34.1%) -- 2.1% chance to 3HKO

So 252 HP/180 Def/76 SpD calm (or 252 HP/76 Def/180 SpD bold) is probably the best spread on average.
source
One way to guess the opponent's moves is going to https://www.smogon.com/stats/ , clicking the most recent date, clicking a .txt file for the format you're playing, and ctrl f for the opponent's Pokemon.

Answer 2

Well, basically, Defense and SD are useless without HP. Defense and SD block attacks better than HP, so ur best bet is to invest in def and SD. See This to help with investing evs: http://pokemondb.net/pokebase/14622/how-do-you-decide-on-evs-for-a-defensive-pokemon