Location, location, location
As stated by Martin Carrara in an article from June 1, 1995, NAPM INsights, p. 52, "The question to ask is not whether or not purchasing should be centralized, but rather which key activities should be managed centrally and which tasks should be eliminated, automated, or decentralized. The argument centers on whether individuals with purchasing responsibilities should belong to a central group or report directly to the various operational units."
Hope this helps you in finding what is right for your organziation. It's a good starting point, I believe.