Visual Studio Web Test vs. Selenium
The major difference between Visual Studio Web Test and Selenium is how each tool works.
Selenium works at the UI level. Tests are recorded and played back against UI objects in the browser.
VS Web Tests work below the UI level. Tests are recorded and played back against HTTP requests/responses.
Depending on your context, one of these tools will be a better choice. If you are focused on functional test automation, requiring the use of the UI, Selenium is probably a better choice. If you are trying to load test, VS is the way to go.