I do not have high activity for any champaign yet. I think pay per post is fairer for the campaign owner, fixed rate for a given number of post is good for the forum, while a fixed rate based on activity alone is unfair for both parties.
While being paid on a per post basis may be good for some, it has the unfortunate tendency to promote spamming - hence the signature restrictions that were imposed on lower activity members to make signature advertising for those who spam (mainly lower level members) far less effective.
)
). If you can post a lot in that period, you should choose this type of deal.