So I have been doing some R&D work on various Physics engines to use for my project. Here is a breakdown of the engines I have used so far
PhysX.Net - My first choice. A wrapper to the in-famous PhyX Engine by NVIDA. It is also the same physics engine used by Unity. It is an amazing engine and I got pretty good results. However, when I tried to run it inside Photon I ran into various problems (typical problems you would encounter using a wrapper). When creating the Core from multiple zone servers I ran into domain issues and the wrapper had some internal bugs which made me give up that engine.
DigitalRune - My second choice. A physics library written in managed C# code. It is a great engine and gave very good results. Unfortunately it requires you to purchase a license for commercial projects. I was given a free non-commercial license. If I am able to finish the project and release it I will be choosing this engine in the end.
BEPU - My third choice. An amazing physics library. The good thing about this library is that it is open source. I got amazing results from the engine performance wise. Its got a good documentation and tons of demos to get you started. I was able to learn the engine in less than a day. It is very easy to understand and work with. This is what I am using for the game at the moment. Since I said that I will (maybe now) be using DigitalRune for the game I decided to design a general purpose physics class in my game so I can change whatever physics engine I need to use at will in the future without modifying anything else other than the physics class.