I think everyone should just accept that there are two ways of spelling her name. Yes I know she was always meant to be called Aerith. BUT, the only version of FFVII that I played labelled her as Aeris. This will be how the majority of Western players (who only played the main game) will know her.

I shall always refer to her as Aeris. However, I will not be offended in the slightest if anyone wishes to say her name with a lisp.