Originally Posted by lordaRaG0n

It depends most of the overall income after all because if the advertiser does not gain much out of his campaign, meaning that he does not gain $xx per signup and gains just $x he will most probably go for CPC which would insure him filtered traffic as only those interested would click.
However if his targets are high and he has a pretty well made campaign with "expensive" outcome he'd go for CPM as it's much more attractive to the publisher although it may produce a lot of garbage traffic.