Sites: Word file or Google Doc?

When I'm using a Google Sites wiki and I've got a document I want to associate with a page, should I attach a Word (or Excel or PowerPoint) file or should I link to a Google Docs file?

• If it's too complex to be a Google Docs file, then it should be an Office file.
• If I want several people to be able to work on it at once then it should be a Google Doc.
• It probably shouldn't be both.
• Any other guidelines?
• How to transition from having everything as Office files on the local server to having Google Docs in the cloud?