If you are like me, I like to keep my Visual Studio environment as clean as possible, so if I can get away from adding any addins in to Visual Studio I will always go for that option.

So when it comes to testing I prefer to run with NUnit and run it as n external program.  

To do this just right click on the unit test project -> choosing the properties option -> setecting the Debig tab -> choose "start external program" and enter the NUnit assembly path, in my case this is "C:\Program Files\NUnit 2.5\bin\net-2.0\nunit.exe", next is to select the command line arguments and point this to your unit test dll,

e.g.  "C:\Program Files\NUnit 2.5\samples\csharp\money\bin\Debug\cs-money.dll" /run

The "run" parameter will automatically run the loaded project