You put it backwards in the title for the argument you are making. That aside, The enton arrow is definitely deadly, but it is not as deadly as a bijuudama. A bijuudama covers a large area, while the enton arrows are focused. The explosion will overall cause more death than an arrow or many arrows.
I do realize that amaterasu can spread, and that pretty much anyone who is hit by that enton arrow will die, but they don't necassarily die from the arrow impact, but the fire, and that takes at least some time (and can be removed by some people, Obito for instance). The bijuudama disintegrates wide spread areas, and vaporizes land (so I assume it would do the same to bodies), and thus kill instantly if hit without sufficient defense.
Without defense against either, you are dead. However with the bijuudama more people can be hit, and overall less people can defend from a bijuudama than an enton arrow. The blast radius is too large with a bijuudama for someone to run away, and most people don't have extremely strong defensive jutsu or bodies, resulting in death. It also takes less aim time.
Enton arrows take aim time, and while it may be extremely quick, it could allow for dodging of arrows or being able to escape when compared to a large group with the bijuudama.
Overall I have to say that when fighting relatively few enemies or one enemy, enton arrows are definitely better, and not necassarily has a higher chance of killing, but less overall energy spent and has much more precision and thus makes it more efficient in killing in small numbers.
When fighting a mass bijuu dama are definitely better and has a higher chance of killing more people than Sasuke's arrow would.
tl;dr
enton>bijuudama in fights with not many people or one person
bijuudama>enton in fights with many people.