Hire externally, or promote/train internally?

Hire the best-fit whether internally or externally with priority for internal candidates.

I agree with Talal to a great extent.
There must be an equal mix of both to ensure that fresh ideas are constantly incorporated and encouraged.
  Most often companies hire only externally or only promote internal candidates, which may not be conducive to the working environment.
Critical positions need additional skills and experience- must be filled by a competent and qualified Candidate, Internal or External.
  Employees should constantly be trained to eventually fill important roles and positions in the company, which will promote a positive Employee Growth environment.