It really depend on your website, some website better in CPM and some website better in CPC. You will never know the result without trying it
Ok. I suggest this: Run a ppc campaing first, let say $20 limit, see how it converts, if you have a click through of +5% then switch to cpm , but also depends on what are you promoting, this is for cpa offers.
What i understand that if you have a high paying content , if you have low traffic you must go for the CPC as i will create more revenue if you have high traffic go for CPA