I have a website (shameless plug: http://www.best-sf.com) focusing on science fiction and fantasy. Every title listed links to both Amazon and B&N for users who want to buy it. I can tell from the hits I'm getting in the two affiliate programs that users prefer Amazon by a good ten-to-one ratio, even though both links are located in the same position on the page.

It's a snowball effect: a site becomes dominant, people become familiar with the brand, that makes them more likely to buy there, which makes it more dominant. What interests me in particular is that B&N's offline dominance apparently doesn't travel well.

Mark