<p>I have seen member functions programed both inside of the class they belong to and outside of the class with a function prototype inside of the class.  I have only ever programmed using the first method, but was wondering if it is better practice to use the other or just personal preference?</p>

Assuming you mean C++, it is always better to define functions
outside
of the class, because if you put it inside the class, compiler may try to inline it, which is not always desirable:
Increase in code size (every object file that includes this header might end up with a copy of the function in their code).
Breaking binary compatibility when function definition changes.
Even with inline functions, it is
usually
better to put definitions outside the class to improve readability of class public interface, unless the function is a trivial accessor or some other one-liner.