considering every time i hear her name in Japanese it's air-ee-su and not air-ee-thu, i'm gonna say Aerith is the correct spelling with Aeris being the correct pronunciation... just the same as Tidus and Zidane sound closer to their Japanese pronunciations of tee-dus and jee-don Aeris sounds closer in my opinion to air-ee-su... of course if i were an American voice actor i'd want to hear the original pronunciation before laying the translation down in stone and causing all of this...