Originally Posted by PXN

With respect to click, you don't get paid for potential click. You only get paid if they actually click. So it really all or none.

Now with impression, the problem is not every ads put up are paid for impression. For example if google put up an ads hoping to get the user to click and this ads and this ads only paid if user click then you will only get paid for click, but if the user don't click on it you won't get any paid for impression since that ads only paid for click.

Since you don't know which ads are paid for impression and which is paid for click it hard to track which is why eCPM is hard to track. I think this how eCPM is.

Generally though, I think if your ads is way are at the top then you should go with PPC. If your ads is down below more than 1/3 down the screen from the top then eCPM is the way to go. This is just what I find more effective but I'm sure it won't be for everyone.