Tried to search but I gave up on the captcha, sorry!

I'm new to the game and I was thinking about what soul to use for my Water team. I mainly wanted a Soul with a class B ATK/DEF debuff, which means Mordred (10% + Overdrive Extension) or D'Art (20% + nothing). Thing is, I have Cthulhu which already has Overdrive Reduction. So is stacking them awesome (and I should go with Mordred) or pointless (and I should go with D'art)?

Oh, I'm planning to be a free player so I doubt I'll ever get Ryu-Oh or some other OE SSR. At most I'll MAAAYBE buy a miracle ticket down the line, and then it would probably just be for SSR Nike or Osiris I guess, I don't know.