Cloud
         "I'm sure she wanted to give her life for the planet..."

Tifa
         "Really? I wonder? I don't think that's it at all."
         "I think she didn't think she would die at all, but that she
planned on coming back all along."
         "She always used to talk about the 'Next time'."
         "She talked about the future more that any of us..."

I think both of them are right. Aeris was the one who talked the most about the future. But maybe she knew she was going to die but planned on coming back somehow anyway, otherwise why didn't she run away from sephiroth?

Aeris

click on aeris for more on this subject
planet can you hear me?