I prefer the spelling "Aeris" as opposed to "Aerith" for various reasons;

1. "Aeris" was the spelling I grew up with, so I feel more nostaliga with that spelling.
2. "S" and "th" are very close in Japanese (I don't know specifics) so either way it's the same.
3. To me "Aeris" looks nicer than "Aerith" and suits the character more.
4. I have a problem pronouncing "th" sometimes, so it comes out as "aerifff" when I say it, so it comes out weird.