There are two versions because, because with for internal use of the physics library.
The physics library was supposed be class less, but internal is C++.
The second one, dMath, is the one Use for tolls and everything else you should use that one if you want.
In general most people using the engine, already has their own classes, all of the classes in the tool folder are very useful.
dMath is the one with Slerp.