I think the flat fee is a fair deal. You can see who you're getting and what types of posts they usually make, and judge accordingly.
The pay-per-post though just invites BS to run up a bigger check. I hate to do this to Netpond, but this is a prime example...
http://www.netpond.com/threaded/showthread...=&threadid=9321
When I looked two nights ago, the last
5 pages of posts were all made by one person. He's getting a nickel per post and at the time was averaging 100+ posts per day. Using such award-winning contributions as "your momma" jokes that he copy/pasted off the web.
The TCG ppp stuff increased this type of posting over at GFY. Rather than the normal 25 replies of

you get 200 of them. And they're all getting paid per post.
In that situation, the advertiser is just losing money *and* causing discontempt for their company through sheer annoyance. I know that "any PR is good PR", but sometimes that just backfires on you.