Both are based on C, which is nice for me because I hate the TikZ syntax. And looking at the gallery for both, they have really impressive diagrams. I have been using Asymptote for a month now, but after looking at some of @Andrew D. Hwang answer's beautifully drawn figures, I want try ePiX.
I am not asking which is better or worse. But, what is the advantage of one other the other? Where does he greatest strength and weakness of these packages lie.