I personally use a mix. I find text ad's work so much better then image ad's when they are placed around the main text of my website.
text ads works better for me, i think it`s becuase there are 4-5 ads in one text unit, while the picture ads has only one ad on them so the visitors cant choose other ads if he doesnt like the picture
I believe it totally depends on the place of Google Adsense. But on average, text ads perform well than image ads.
I find image ads to be much better. They have a better chance of being clicked on because people don't know whether it's an advertisement or not. Also image ads have a higher CPM, at least for me...
I think text ads more profitable, because most Internet users will open the page after reading the information in these text ads.