.Net forces the programmer to define which functions may be overriden, whereas Java functions, by default, can be overriden unless the final keyword is used.
If you're a strong advocate of the Open/Close Principle you may tend to support the Java way. It's best to allow classes to be extended and methods to be overriden such that the base functionality/code is untouched. For this reason, I support/prefer the Java way. If I were looking at the question from a different perspective, my opinion may be the opposite.
