We've tested both and decided to go with WaTiN. As others have pointed out, Selenium does have some nice features not found in WaTiN, but we ran into issues getting Selenium working and once we did it was definitely slower when running tests than WaTiN. If I remember correctly, the setup issues we ran into stemmed from the fact that Selenium had a separate app to control the actual browser where WaTiN did everything in process.
