Why not use both ! I will definitely go for Text ads because of the high CPC . In the case of link units the earnings can vary a lot .
I accept both image and text ads. While text ads give more choices at one time. Image ads change the look of the web page, giving it more pizzazz from time to time. Of course, I like graphics and use them whenever it is sensible on my pages. Honestly don't know if image ads get clicked more. But we are a visually oriented species and a lot of money and time is put into creating some of those image ads to get people to click on them...
If I had to pick one it would have to be the text ads...but you'll only find out if you test out the ads and see for yourself.....