Picking one best singer is a very subjective thing, but both Justin Bieber and One Direction seem to have about the same level of talent. Both make good music that appeals to their desired fanbase and both have found commercial success with their music. They appeal to slightly different groups so it is harder to pick which one is better.
They boys of One Direction and Justin Bieber both have wonderful singing voices, thus making it too hard to say that one is better than the other. One Direction can produce accurate harmonies, while Justin Bieber is a solo performer. Since these are two different skill sets in the vocal field and each have equally enjoyable songs and performance moments, there is no way to say that Justin Bieber is better than One Direction.
While the question of who is better is highly subjective, it's safe to say that Justin Bieber is neither better nor worse than the boys of One Direction. They both have lots of talent, and their music is edited to sound better when you hear it, so there's no way to truly tell who is a better singer.
Justin Bieber and One Direction may appeal to different audiences. At the end of the day, however, they are both teen pop sensations whose singing style is heavily tailored the tween and teen demographic. There sound, as a result of this, is very similar and to choose one over the other is akin to choosing Hunts ketchup over Heinz. You may prefer one slightly over the other. But at the end of the day, they are both ketchup.