I've been questioning why I should move from WatIR to WatIN. There will always be a divide on Open Source Tools, and some Testers will go with Selenium, some with WatIR, and possibly some with WatIN. I'm not sure if everyone will ever agree, and its obviously a question of what you need to test.
There's clearly a number of Pro's & Cons for each tool, but personally, I'm going to opt for WatIN! Out of the box, I can get my tests running under IE & Firefox, without the need to install an additional IDE for the Browser. The biggest downside to WatIN is that the Community doesn't appear to be anywhere near as big as WatIR. Moving from Ruby to C# isn't as painful as you might think, and with the help of the.NET Developers I work with (nobody seems to like Ruby here(except me)), I've been able to pick up .NET fairly quickly.
Within a matter of a fortnight, we've got out tests running with NUnit & Firefox with what seems to me as 'simple' code changes.