Each way has it's benefits and drawbacks:
One Piece: Less string, only two knots, some say it looks more "professional" BUT: A ton of string to pull, sometimes you have to think through ATW
I don't think performance has anything to do with it. Both do the job. I like two piece because, to me, it's easier, it's the way I learned, I tie a good looking knot
, I don't really have to think about it, an especially when stringing gut, it's much easier on the crosses because you don't handle them as much.
Tod