Discharge : iron will vs controlled destruction

What is the best to use with discharge? In several builds people are using controlled destruction, but from what is it written iron will is largely better : there is no crit reduction and spell damages are quite the same but it seems different, can you explain it please?

Iron will is mostly useful for builds that utilize lots of Strength.. so what will be better depends on your build. There is significant difference in these supports wording, controlled destruction has "more" damage, which is standalone multiplier. Iron will offers "increased" damage which stacks additively with all the other applicable sources of increased damage

The difference between the two is that Controlled Destruction grants More Spell Damage, and Iron Will grants Increased Spell Damage.

More Damage stacks multiplicatively with every other damage modifier.
Increased Damage stacks with other Increased Damage modifiers (from tree, gear etc.) and thus will become less effective the more you have of it.
http://pathofexile.gamepedia.com/Stat for more information on this.

In short, More Damage is always either equal to or better than Increased Damage.

It depends on the other "increased" and "more" modifiers you have. If you have little to no "increased" modifiers and lots of strength, then go iron will, no questions asked. Otherwise controlled destruction. Also, socket colours might be a reason to use one over the other.

Oh ok i see, thank you for your answers !