it all depends on the niche...some niches can yield 20-30 percent ctr with a quality layout. however some niches can only yield about 5 percent. Its really all in the niche. Also, some say that text ads are best because they are most targeted. Lots of times image ads have nothing to do with the site. For example, a site about light bulbs and then you have a big image ad for insurance. With text ads they tend to fit your site more.
hope this helps

Keep changing position to test which got best performance and pop out ads usually for those less quality site but with lots traffic.