I've used both, they both seem to work ok. My nod is for Selenium as it seemed to have better Ajax support. I believe WaTiN has matured though since last I used it so it should have the same thing.
The biggest thing would be which development environment do you like to be in? Selenium and Watin have recorders but Selenium is in the browser and watin is in visual studio. + and -'s to both of those.
