Cordelia. She willingly put aside her own life, her dreams for the future, and willingly took the pain of having visions all in order to do good and help people. She didn't have to do that. She could have moved on and had a normal life, but she showed how much she matured by deciding that other peoples lives were more important.
For Buffy, they just threw some angst at her to make her miserable and called it development. Like someone else said, she was simply reacting to the events around her, whereas Cordelia had choice in her development.