Hello Hugo ,
I work on this field so i'll give you a fair advice : Don't think that paying less money on 1000 impressions could bring you an enourmus CR. Go with the CPC for the moment ,it's a lot better than CPM .
Also , if you want - you can see for yourself . Create 2 campaigns : one on CPM and one on CPC and see whom works better .
Good luck with your business!