Most features should work equally well with MSTest, NUnit, and other test frameworks.
The Pex team uses MSTest as the main test framework for its development, so it's simply best tested, and new features often become available first for MSTest.
A particular feature area where the test framework experience differs are Moles: While MSTest has the concept of a "HostType", which allows us to run a test in a separate process under a profiler to redirect the calls as required by Moles, most other frameworks don't have this concept. We are currently working on making Moles available under other test frameworks as well.
Nikolai Tillmann -
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