title: What's the difference between the .NET Unit Testing framework out there?
body: <p>I've heard many different opinions as to what makes one better than another. But can anyone provide a technical explanation as to why I should choose one unit testing framework over another. For example, I use Visual Studio Team System. Why would I choose an Open Source testing framework (NUnit, xUnit, MbUnit, etc) over Visual Studio's built-in unit testing capabilities? Does it matter?</p>
  
  <p>Edit: To be clear, I'm not looking for an opinion on "Which unit testing framework is best"...I'm looking for technical details as to how they are different,.</p>