Advertising: why I'm going with cost-per-impression rather than cost-per-click … but am I being stupid?

I’ve got two online ad campaigns running. One is based on cost-per-clicks (how many times someone clicks on it and goes to my site) the other is a cost-per-impression (how many times it appears on computer screens across the US). Both are targeted to specific demographics. 

The cost-per-click ad is getting a higher click-rate-per-impression than the impression ad, and my first response was: “Awesome! They’re going to my site!” And my first instinct was — that when I suspend one of the campaigns, because I can’t afford to keep them both running indefinitely — to keep going with the cost-per-click model.

But then I realized the other ad is getting almost triple the number of impressions for the same price, and so I’m going to go with that one, instead. 

Clicks are awesome, but I’ve realized that my book-buying patterns aren’t really all that impulsive (and will probably become less so as my belt tightens in this economy). Instead, I’m worn down by repeated mentions of a series or an author’s name, so that when I’m at an online bookstore or a brick-and-mortar, I’ll pick up a copy and look it over because, “Oh, yeah, there’s the one I’ve been hearing about.”

And all other things being equal (covers, titles) I will pick up the book that is familiar in some way, even if I’ve never read the author before. 

But maybe I’m not a typical buying model? Of the past ten books by new-to-you authors that you bought, did you buy more books impulsively (not knowing anything about them, but liking either the description of the books or the cover) or did you buy it after hearing/seeing the author’s name at least a few times?