Impossible to say whether an internal or external manager should be employed without having a lot more context and understanding of the environment and scenario at hand. Stereotypically an internal manager may be capable and would have an excellent knowledge of the organization. This person could serve as a tactical manager for the “Owner” or senior manager responsible for achieving synergies for the organization. Unless an organization routinely acquires companies or has an M&A department, I would doubt an internal resource would have the level integration experience that and external candidate would have. The average Director or VP may see an acquisition or two in their careers whereas a “professional” Integration Manager earns a living managing nothing but PMI. The external candidate should be much more seasoned that anyone internal.
Understand that I am making some assumptions and as I indicated initially, some context and understanding of the integration would be needed to properly answer your question.