Kind of simple answer too, but it depends on whether you get a lot of traffic or a lot of clicks on your chosen ad placement. Some sites naturally lend themself to CPM - things like proxified pages for example. Trial and error is where most people start off.
I combine the two, usually the highest paying option is displayed, with avnads, this way if there's no one paying PPC for second rate countries - then hopefully someone is after CPM in the country instead.
I've tried a bunch of CPM ads with zero results. In contrast I have had excellent results with PPC ads. I look at it like this. CPM ads are like a billboard ad on the highway. You get thousands of people zipping by who might glance at it if you're lucky. Only a small percentage of those people are actually interested in what you're selling. An even smaller percentage of those are going to buy anything. Strictly a volume proposition. Granted if you can get it onto a very relevant site you could be successful. With PPC ads you only get people that are interested in what you are selling right off the bat. Not only are they interested but they are actively looking for it at that very moment. You can be very specific with your keywords even to the point of "buy <product> <manufacturer>". How can that possibly compare to the billboard? mp :8^)
I also had the same question, but you guys answered it so well Although I have an extra question, what if there is only a very small audience for your product, would PPC also be the better option?
it would seam that the OP asked as an publisher,not as an advertiser. i will answer the question for the publishers and advertisers: it depends. any more questions,feel free to ask me.
I have found that if I am offering something for free or if Ihave a good ad that creates curiosity I get more for my money with CPM.