Superiority has nothing to do with who would win. Sasuke had an advantage that Minato wouldn't have. Sasuke's eyes was able to see the effect the first time he used it, and that's why he used his sharingan to trick Danzo. Minato doesn't have that advantage, yes Minato is far superior in many things than Sasuke, but this is a fight of the sharingan, of genjutsu. Minato can understand the technique the first time he sees it that is if he escaped, and if he does escape than Danzo would be in trouble. But Danzo is an old school shinobi, he's also smart, more knowledgeable, more experienced than Minato. Minato's speed is amazing but would little to genjutsu. Minato's best chance would be to move first and be done with it, but Danzo knows of his speed as he was renowned for, so Danzo would use Izanagi first.
Danzo beating Minato wouldn't mean he's stronger, but had an advantage. It's the world of ninjas, power alone doesn't always win you a fight, but skill, intellect, strategy, tactics to be a step ahead. Naruto is stronger than Sasuke now, but if he fought Danzo he would have lost.