To be honest, their stun rate is approximately the same. Since Thucydides is a blue hero he's often starred and sainted higher than Cianna since it's cheaper to do so which might be why it seems ol' Thucydides stuns better.
Regarding the differences in what these skills, Dizzying Strike hits the target first in line, whereas Ambush targets the last (ie the hero/monster at the back).



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