Criteria should be the context (types of tasks, urgency, organization constraints...) and the staff ability and commitment. Now in practice, when I arrive in a team I must admit that I follow a bit the implicit structure that is already in place; I have always been with staff that are already competent, and that are dedicated, so I tend to have a management style based on delegation (I think because it is difficult to distinguish from distribution of tasks). So I would say that the main criteria I use usually is the ability of the colleague.