Due to it's popularity and active development, plenty of learning resources (such as detailed documentation and various tutorials) exist for learning NUnit.
NUnit is one of the most popular testing frameworks for .NET. Other than giving a certain sense of security in the continuation of the project, it also means that there are a lot of third-party resources, guides and tutorials available for NUnit.
It has more readable Assertions out of the box like Assert.That(myClass.MyMethod(null), Throws.ArgumentNullException.With.Message.Contains("param");
NUnit also has good tutorials in using the variants for parameterized tests: e.g. its easy to find an example how to correctly use [TestCaseSource(typeof(myTesCaseEnumerator))].