Banners can perform quite well, if they're targeted to the audience and/or the page topic. (Most banners perform poorly simply because they're run-of-network garbage, such as the mortage and dating-service banners that one sees on newspaper sites.)
In any case, the question is probably academic for most AdSense publishers, because so far there doesn't seem to be an overwhelming demand for image ads. I expect we'll see more if and when Google succeeds in marketing AdSense to large mainstream corporate advertisers and ad agencies.
I never tested it in that amount, that I could make a reliable statement.
I expect that text ads perform better, because they canbe better integrated into the content and aren't highlighted that much as advertisement as Image-Ads are. Regular image ads normally suffer from banner-blindness, too.
Due to the better matching I expect that Image-Ads on Google perform better than Image-Ads from any other regular advertising network.