My personal experiences with Dark Physics have only been reasonable when working with rigid bodies, static collision meshes and joints. It also features a library of example projects whereas the Dynamix example files do not cover much of the commandset.
Other than that the vehicle, character and general purpose functionality of Dark Dynamix are much more reliable and consistent.
The only reason I would opt for Dark Physics is if you want to use something with lots of example files or cannot get Dark Dynamix to work with the open source DBP-GDK; you can obtain a free copy of the limited 'lite' edition of Dynamix
here.
Be aware that there are other forum searchable collision or physics libraries such as Nuclear Glory and ODE physics available.