Advantages of Powershell for web-ui testing?
Powershell will give you advantage if you need to manipulate something else than a browser/webpage/webapp. With Powershell you can do something with OS, applications, things outside the browser.
If you don't need to do that, if you want to automate only web based app, than I would suggest using web specific tool. Selenium is nice, tests in MSVS2010 are great. If you prefer coding try WatiN for .net, and WatiJ for java. All of them will run smoothly not only inside IDE, but also on CI server.