It's not that Aeris' death is more shocking, it's that FFVII is more mainstream.
I personally believe Kefka's action greatly outweigh Sephiroth's but that's because I've actually played both games. Aeris' death was moving, but as you said, Kefka killed an entire nation because he was impatient.
And if you think it was just nameless face, read this and realize why that is such an incredibly emotional sequence.








