I've ran tons of campaigns in many different niches, and have never seen image ads do as well as text ads. The click-through-rate is always lower.
However, some sites only run one type or the other, so try both. You will reach different audiences.
Also, experiment with content network vs search network. In some niches, they are comparable, while others the search network will outperform 10 to 1. The good news is content network is often 1/4 to 1/10 the price of search network.
Always split test, multiple ads, multiple networks, even rotation, then compare results and fine tune.