When Google came up with their text ads, they were very successful because users seemed to click through at a higher rates over image ads. It was my belief that text ads would get a higher click through rate thereby increasing my earnings. In many of my ad settings, I had gone with text only ads but had to reconsider the setting based on the following email I got from the Adsense team.

Our system analyzed your domain, dinesh.com, and found that you€™re not opted-in to image ads on at least 50%  of the pages of your site. For example, we€™ve found that your 160x600 ad unit on dinesh.com/history_of_logos/worlds_best_brands_and_logos.html is only opted-in to text ads. We recommend that you opt-in to image ads for all of your AdSense ad units.

We've found that our publishers who show both image and text ads are able to earn higher revenues. Opting into text and image ads increases competition among the ads competing to appear on your site, as both text and rich media ads (image, flash and video) will compete in the ad auction. We will then automatically display the ads which will yield the highest earnings for you.

I did change the setting and what I found was that with the image/text setting, my click through rate dropped a bit but my ECPM went up thereby my earnings went up. I would strongly recommend experimenting with this setting.

Observe data for a week before and for a week after the change and make a decision. Each site is unique so you may get different results.