For any savant, we need either Conjuration or Evocation focus giving +1 (to +3 with greater and epic) to spell DC.

For damage spells, as far as I saw, the DC is used in spell sometimes to bypass all the spell, like niak ray (L1) or half effect for most AOE spells.

But it seems that hight level damage spell don't use any DC at all, like niak bitting cold or frost ray, even AOE Ice storm don't have DC to avoid damage.
The first frost spell, Niak's Cold Ray miss entirely if DC fail, but this spell is Conjuration.

I read a lot of thread on sorcerer and I'm looking for a water or earth savant.

In both case, I conclude that Evocation is more interesting.
Earth spells are almost Evocation, and the ones using DC to resist are Evo.

Frost high damages don't have resist DC, frost lance have but don't scale on level so Polar ray is quite better.
There is no good CC spell in frost so bit DC is not needed.

Earth in comparison have same AOE spells (fireball like) so using Snowball or Acid blast is equal, but earth have cloudkill or flesh to stone.

Other good CC spells are illusion or necromancy, so taking evo or conju, don't help

For a sorcerer build, with so few spells we need to only focus on high damage spells or most usefull and I can't really see why Evocation is better.

Don you have another reason to take Evocation over Conjuration for Sorcerers ?