Special Attack bought with physical bulk. Modest gives up less for the same power.
The multiplier lands on the final stat — ×1.1 on the one it raises, ×0.9 on the one it lowers — so it's worth more in absolute terms the higher that stat climbs. HP is never touched by any nature.
Every non-neutral nature likes the flavour tied to the stat it raises and dislikes the flavour of the stat it lowers. This is what drives berry reactions, Pokéblock and Poffin results, and curry ratings — and it follows the listed nature even if a Mint has changed the stats.
Ranked purely by the gap between Sp. Atk and Defense in their base stats — the wider the gap, the less Mild costs them. Base stats only; this ignores movepool, ability and format.
A Mild Mint applies this same spread to any Pokémon without changing the nature it displays. Berry flavours keep following the original nature.
Impish runs the same two stats the other way round — +Def −SpA. Worth a look if you picked the wrong side.