Sorry for being vague - I was talking about ad formats on your site as a publisher not an advertiser. Earning money instead of spending money in this case.

It's my understanding CPC pays more if you get the right ads, but CPM is better if you have a lot of views without a lot of clicks or high-level advertisers that will generate clicks for you ( i.e. CPM is better for a flash games site or a videos site where lots and lots of people come to see your stuff).