I suppose that since there are so few songs that aren't Yuuka, a better question is what differences do you notice in style, and what do you like best about each?
As for Asuka, I like how her voice sounds nonchalant but still straight-on, like it comes very naturally to her. The strings in the songs with her sound more like a full orchestra than solo instruments, giving a warmer sound. And Keiko...her voice is definitely deeper, but I don't have as much to say about her. XD