Kojima's style is different than Amano's. Of course both are really talented, and Amano is a legend, but something i noticed about Amano, is that he doesn't use shadings a lot. It's as if there is no shadow in his paintings. Also he uses watercolors , which makes his paintings look so bright.
Kojima focuses on rendering flesh tones.As far as I can tell she uses acrylics, which are closer to oil paint . Plus her influences are totally different than Amano's. She likes gore, surgery and she was impressed by the effect produced by oil paint when she was studying arts in high school. She created her own style that includes these elements and it works very well.
For me the comparison is vain, since both artists are good and they have their own styles. Whether you like Amano more than Kojima, it's just a matter of taste really. Amano is too fantasy for me. I like Kojima's gothic/disturbing art better.