I dont want to start a flaming war, im am an open source "person" always have been, i advocate it in my notes and in my programs. DirectX and OpenGL can''t really be put on the same playing field. The movement for OpenGL is a massive one, but microsoft has the ball on their side, considering they''re the ones who made the Windows OS. So they know more about their code and framework. OpenGL has great benefits, it is multi-purpose (not just for graphics) and if you do your graphics in OpenGL the chances are you will use DX calls for networking or sound or something or other, until OpenAL become big and a dominant open source game networking API comes out, you''ll be on your own. I have nothing against DirectX nor OpenGL, it is all about what you want to do, but personally OpenGL calls are simpler and i like it better, but when i become more experienced i probably won''t be so ignorant. Make yourself a better programmer by using both, don''t take sides and keep your mind open to different things, good luck, and if you want to learn more about these two API''s there are dozens of flame wars in the forums.

Have a look at :

Direct3D vs. OpenGL: Which API to Use When, Where, and Why by Promit Roy (GameDev.net)