I see Yamato winning. While Asuma is an expert in close range, i don't know if he can keep up with a long range fighter. I mean, i was really surprised that he could not even keep up with Hidan blade, and he got wounded really easily i think. Yamato could overwhelm him with his Mokuton. Don't also forget that Yamato is very cautious when he fights. He won't run toward his opponent without thinking. He can also use clone Mokuton, suiton, and doton to make an approach.