Don't get stuck on choosing a framework. Just pick one and start testing - they're not all that different. When you have written tests for a while, you will know what to look for, to suit your needs.
Personally, I have found xUnit, Testdriven.Net and Moq to be a very flexible set of test tools.
Also see this post: NUnit vs. MbUnit vs. MSTest vs. xUnit.net
