I'm from America, played the North American PS1 version of the game, so I say Aeris and choose that.

BUT, I've been on enough FF message boards in the last 13 years to notice that I catch myself calling her Aerith nowadays as a habit because so many gamers go by that name of hers world-wide. So as much as I hate to admit, I catch myself calling her by the name opposite I believe more times than not.