They are NVidea physics wrappers, providing you with most of what you need to create dynamic scenes quickly. The difference is commandset and examples provided.
I prefer Dark Dynamix as the main physics engine, given its more frequent and recent updates. The creator is very helpful and actively communicating with his customers. This one is a bit more professional in structure and function; given its more practical function and properties.
Dark Physics is a good product which includes a more comphrensive documentation and example library. It has a number of incomplete functions and a poor quality vehicle engine; but its library provides a solutions for many situations.
I use them both for different aspects of my system; I recommend trying out them both to find what suits yours. If that is not possible, Dark Dynamix is the one I recommend.
