Whatever gives you more toughness, as simple as that. Mitigation from allres and armor is calculated multiplicatively and works the same, so whatever increases your overall mitigation (observe toughness changes while keeping other stats the same) is better.
If you're talking about from Plvls, bear in mind that armour is a % increase, so if you have a low base armour (e.g. you're not a str class) the increase will be minimal, whereas the all res is a flat increase.
No man is sane who does not know how to be insane on proper occasions.
id go with all resist