First, I imagine that it would depend on the base stats of the pokemon.
Assuming the base stats for the pokemon, like mew, are equal on HP, DEF, & SP. DEF, I used a damage calculator between mew and an alakazam.
I maxed Alakazams SP. ATT. and Speed EVs at 252 each. When I put 252 HP EVs and 0 SP.DEF. EVs on Mew I got an 18.8-22.2%HP taken per hit. When I had 0 HP EVs and 252 SP. DEF. EVs I had 17.5-20.7% HP taken per hit. Alakazam used psychic for each turn in this test. No items were held. I made both of their natures neutral, no held item.
In this scenario maxing the defensive stat is more beneficial. This is in no way conclusive or applicable to all scenarios, but it is a baseline to start from.