Image for me. I recently changed formats to see what would happen and while both kinds of ads seem to have nothing to do with my content, whichever ad people actually click on doesn't show up in text ads.
I earn highly from both. IT is dependent upon your sites layout and setup, there is no one size fits all for every website.
According to me,Image ads are better than Text ads since Image ads one click more money than single clcik on text ads.