Don''t forget one thing : openGL was for long time and still is used as a PROFESSIONNAL library. OpenGL is for example used in maths labs.
...but, DirectX ? not at all, directx is not that kind of library, it does only game-oriented programs, and it is not portable.
I also prefer OpenGL. DirectX is in version 7, OpenGL in v1.2. But much new features of D3D have already been in OpenGL for long time, like T&L. So older OpenGL games are accelerated by T&L of GeForce whereas old D3D games are not.