She's most likely the greatest hero in the game. Aeris knew of what she had to do, and only SHE could deal with it... that would be an amazing human quality to posses. Expecially since she was always looking on the bright side of the journey and trying to cheer most of the people.

What did Tifa have? To be worried that Aeris would steal Clouds heart? Heh, that's a real burden... :rolleyes2

At least that's what I think.