Some of this depends on available resources. If you have resources internally that have the capacity to fill this role, they can usually ramp up faster because they already know the business. On the other hand, if the company assigns this internally and the person does not have the capacity to take on the work, the integration will not be very successful. If the company does not have plans to have future M&A activity and does not currently have internal resources, it makes sense to bring someone on externally for the duration of the integration.
As stated by others, there are certainly pros and cons to both. From what I have seen, internal managers have been the most successful. We have had a few external integration managers and they have either been let go part way through the project due to a mismatch in personality and culture fit, or they did just as well or better than internal staff and were offered full time positions with the company.