Aeris is a better name than Aerith. It sounds like the real Greek name Eris. Aerith sounds like a lisp.
Both names are the same to Japanese ears, so we should be allowed to pick which one sounds better to our English ears. Final Fantasy is full of names that got messed up like that in the Japanese-English roundtrip. Barret was supposed to be Bullet. Rydia is probably Lydia. We have no idea what Ogre Nix/Ogyrenyx/Organics is supposed to be.
Nomura has no idea. We, English speakers, should be allowed to pick the names we like better. It's all a bunch of made-up fantasy nonsense, anyway.