Def or STA?

I would always take attack + stamina, in defense and in attacking.

if potion usage is an issue then defence because it'll reduce potion usage slightly. I doubt it'd be much. and even then having more HP on attack is still useful because losing HP = gaining energy. you're probably using hyper potions and overhealing anyway.

besides low def pokemon like chansey and wigglytuff, which really benefit from the extra defence, most pokemon would have similar gains in EHP from defence as stamina, but defence is less versatile, like you said the HP bonus from stamina is doubled on defending pokemon, and it's not like you're never going to put your attacking pokemon in gyms.