If people try both methods, they will generally feel like they shoot better either 3 under or split. Since archery is such a mental game, feeling better about something is worth something in itself.
Various coaches or archery instructors feel like there are advantages in one over the other. For example, Rick Welch teaches that 3 under is preferable to split, as he feels that it eliminates certain errors that are peculiar to split. For him it is a preference, not a really important component of his method. Other coaches, such as Arne, are neutral on the subject, while others, such as Fred Asbell, teach split.