My short answer is both have their pros and cons but would need to understand the starting perspective to accurately answer your question. In my opinion, an internal integration manager has one key advantage(pro), that is they are part of the organization and could have final decision power. In contrast (con) they could be myopic in their view of things and influenced by the “victory syndrome”. An external consultant IMO manager can only make recommendations, they cannot tell someone to do anything or approve expenditures or personnel time/project changes(con). However, they may be more objective about the entire IM project (pro) and could avoid making decision/recommendations that are biased.