What gets me is between the two, I'm pretty sure Seven is the more desirable unit? Origin games aside, she's the one with party benefits, including brv and hp damage up, as well as follow-up attack after literally anything happens
even comparing him to Seph doesn't feel quite right; the only thing he really has in common with him is that his EX has a 100% gravity/free ability use effect
Kadaj is Seven all over again, but people forgive him because he's a Sephiroth/he's not a Type-0