Pay via "Per Click" and "Per X amount of Impressions"? I am yet to use any of these advertising programs. However I plan to go with AdSense eventually.
Most of the Adsense ads pay per click, but they are also so me ads that pay by impressions (very little). Adsense chooses automatically which ads to show, you don't have control over this.
I think most of the "Text ads" are Pay Per Click and most of the "Image ads" are Pay per 1000 Impressions.