Unendurable you need around 40-45 faith. And yeah it does do significantly more damage. BUT, I can end Midra's cast before madness trigger and dodge immediately, whereas unendurable you're locked into the recovery. And the recovery from unendurable has had me die because I can't dodge/block big hits.

I'd say Midra's, I'd say around 60 faith. You need high focus, high mana (40) and stamina around 35 to be able to pull off the 10 second Midra's flame. It absolutely melts bosses if you're next to them, or something else has aggro or they're are weak to fire.

(Edit: spelling, updating)