direct x or opengl

Depends on what you want to do. DirectX has the advantagethat it's not only interfacing Direct3D but also DirectInput ,DirectPlay etc... While OpenGL is just interfaces 3D graphics. DirectX is updated more often then OpenGL. OpenGL is very outdated on the current Windows OS's. But the new library is coming very soon I heard :) Both API's have a large comminuty to learn from. But I think with C# DirectX is better to learn because the DirectX9 libraries are managed ,so easier to access then the unmanaged OpenGL API.