Which type of campaigns are more cost effective in PPC marketing?

Question

Hi,

I want to know a very simple question that whent we participate in Sponsor link AKA Pay Per Click plans such as Yahoo Sponsor Search and Google Ad Words, which provide sponsor results in many search engines and directories, they usually provide CPC (Cost per Click) and CPM (Cost per Impression) model. Am wondering that from these two model which was is most effective.

I would appreciate if any professional tip can be provided with clear difference for both models.