I realise that there are many older questions addressing the general question of NUnit v MSTest for versions of Visual Studio up to 2008 (such as this one).
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I've mainly used NUnit, some xUnit and some MSTest. They seem functionality equivalent, but I don't like the MSTest test runner. It runs in visual studio so it either crowds the screen or is on another monitor getting in the way everytime I tab to visual studio. (I run NUnit on another monitor, but it doesn't cover everything on that monitor everytime I focus visual studio). It takes too many clicks to find out what test failed and why.
NUnit can run in the background until a test fails, at which point it shows you information about the breaking test. This seems like the ideal for keeping red/green/refactor going smoothly.