Yup, agree with the above.
You could argue that a player who had clicked through from a text-link is more ‘informed’ and likely to convert as they have almost certainly read the passage of text and been convinced to click the link, as opposed to an ‘impulsive’ reaction to clicking a banner.
Also, think about the amount of advertising we all see on a daily basis – whether it is TV, billboards or even on news sites like the BBC. The average consumer is a lot more savvy these days and is adept at tuning out those message which do no immediately appear to be relevant.
Therefore, ‘relevance‘ is the key. As Chris says, the market you’re chasing is an important consideration. However, many affiliates may find greater success with a text-link embedded in a relevant article, which is SEO optimised so that when they google a subject YOUR pages, with their text-links, appear.
Why not run a comparison test to see what type of market you are currently attracting to your site?