Hiring Technical Leaders: Promote from Within or Go External?

Alternative title - Hiring Technical Leaders: Build or Buy? When it comes to (technical) people leaders, should we nurture and promote from within the organization or hire from the outside? What is the not-so-hidden cost of both of those approaches (fresh ideas, different approaches to people and technical leadership, leadership maturity vs organization maturity); technical leadership - building credibility within a team; leading at different sizes and different maturity of organization - the role of a leader in a start-up vs scale-up vs large organization.