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xxMATTGxx;
The posting competitions in the past haven't been ongoing like this initiative. After a few days posting generally dies down and only a few users continue. If you offer it every single week then it allows everybody to get involved as and when they please. They wouldn't necessarily post aiming to receive the rep, it would just act as a nice reward for their contribution at the end of the week.
stick to rep though, no free vip/donator because people just go wild for it and will definitely post stupid threads.
Like you said, it's proven to work well for staff, what's the harm in trialing it on the norms? ;)