to try and weed out the "it depends on XYZ" type of answer, i'll go this route: if you're app has a lot of pageviews associated with it, go with CPM. if not,

to try and weed out the "it depends on XYZ" type of answer, i'll go this route: if you're app has a lot of pageviews associated with it, go with CPM. if not, CPC. figure out your average CPM or CPC rate for your adsense blocks, and double it. start from there -- if you fill up your space easily, increase prices a little each payment cycle until you're comfortable with the income. don't get greedy and make sure the customers are happy.