"Aeris" is a mistranslation, as her name was meant to be a katakana transliteration of the word "Earth." "Aerith" is as correct as you can get, despite the fact that it technically can't exist when romanizing due to the pronunciation being "Aerisu," which is what caused the problem in the first place.

Pronouncing it as "Aerith" is retarded. If you're going to do that, just call her "Earth." I pronounce it "Aeris," personally, because that's the original translation as well as the closest American adaptation of the Japanese pronunciation.