It depends on the situation, but i think Amaterasu is more avoidable since it's more predictable. To use Amaterasu, blood must come out of your eye before in the contrary of Kamui which is more like an instant jutsu(he warped arrows in an instant). Also, since it's small, it's difficult to notice
Kamui IMO since it can really only hit you if you're moving in a linear way. With amaterasu one must have insane speed to avoid it.