Buying Advertisements: CPM vs. CPC ?

Hi,
If someone is going to buy online advertisements, generally speaking, which one is better to purchase…CPM impression advertising, or CPC cost per click advertising?

My personal opinion from my brief experiences would have to say and prefer CPC advertising.
Example: I bought some Facebook CPC ads. I had like 5K impressions, and only 2-3 clicks. @ average 20 cents/click. So total cost is about 60 cents.
If I were to buy CPM ads, then it would have surpassed 60 cents easily for those 5K impressions.
Thoughts? supporting or denying one or the other?

See original post by corgi