While it should create a lot less spam, the cost per impression would be higher for Stunna.
Those existing campaigns pay about 0.1 btc for 50 posts, and so Stunna will probably need to pay the same rate as well (=0.002 per post if everyone make exactly 50 posts), which will be a lot higher than Stunna 0.0006 per post rate in his mind.
https://bitcointalksearch.org/topic/m.9291093
Even assuming that "cost per impression" is as important as you make it sound, you are really stretching the math here. Most people won't post EXACTLY 50 posts, that's the point of fixed rate / minimum post rule - it ensures a certain amount of minimum exposure and anything above that is free. It also doesn't have to be 50 for 0.1, the rules can be set up any way that makes sense. I don't think there would be a shortage of participants if the minimum is higher than in other campaigns. Set it to 0.1 for 150 minimum posts and you'll have ~0.0006 per post plus free advertising on top of that.
TL;DR: FIXED RATE FTW.