The first one can only be used inside IDE, launched by testRunner.
NUnit Silverlight is intend to replace the default MSTest with NUint syntax in default Silverlight unit-test project ship with Silverlight tool-kit. By default, SilverLight UNnit Test project from Toolkit comes with MSTest, it can be replaced by NUnit, by referencing NUnit SilverLight project.
To make it continuous-build friendly, I had to create a local lib folder right under silverlight unit-test project then create those linked reference, otherwise build server kept complaining missing reference error.
RhinoMock has a SilverLight edtion in case you need to mock something.
This walk through explained how to use MsBuild to implement a command line runner, note, must navigate to test project path to launch MSbuild command, not the web one.
SilverUnit is only used by TypeMock, a commercial product.