It's hard to compare... it depends on the distance and let me tell you why: amaterasu forms where you focus your eyes on, and it doesn't come out of your eyes and travel there, it forms there meaning that the distance is irrelevant, while the arrow has to travel that distance.
This being said if the enemy is close enough, and Sasuke already has the arrow formed and the bow tensioned then i think it will reach the target faster than the amaterasu... otherwise (if the arrow isn't already formed, or if the target is further away) amaterasu should be faster.