For CaptainNoobCN's situation, replacing Diana + Sniper Shot with Cthulhu + Ambush would increase normal attack damage by 10%. That's assuming all debuffs land; the lower the base level of DEF down, the less advantage additional DEF down provides.
Replacing Diana with Dark Amon, assuming Sniper Shot is used with both, would result in a tie. That's a bad idea because unlike Cthulhu, Dark Amon doesn't bring additional utility to the team, and just has lower HP.