I'd hardly call FFVII the test of a "True FF Fan" since FFVII and VIII were extremely deviant from what most Final Fantasies are.

Anyways,

With the hard stats, Safer Sephiroth does come out on top, but that's assuming that Fallen One doesn't work, and also how they are being controlled. If it were both in control of A"I" then it'd be up in the air most of the times. The Super Nova/Fallen One is the big issue, IF they work more often than not Kefka'll come out on top. If they do not work, then Safer Sephiroth probably'd win most of the time. Although come to think of it, I don't believe Safer Sephiroth can do a whole lot of damage with his attacks, as I recall correctly he was never getting over 3,000. I don't recall the damage that Kefka was dishing out, I'll have to beat VI again before I can do that, so I still can't say with absolute certainty who does have the edge in this fight.