That doesn't really have anything to do with it. It ultimately comes down to what OSes you want to support for your game or game engine. You can use DX or OGL for games or game engines, but OGL is supported on pretty much all OSes while DX is mainly Microsoft only devices and computers. Both have steep learning curves, but are both well documented last I knew.
I was replying to jsbrown, I didn't mean to imply either was a game engine. He was talking about using a game engine instead of OpenGL/DirectX directly. I'm sure Gollum was talking about the same thing.
One thing you may consider is that most non-gaming applications use OpenGL.
Then again, you may need to know both DirectX and OpenGL to work in certain parts of game programming. And if you work for Windows, they will probably assassinate you if you use any technology they are in competition with.
I agree with most of the conversation going on here. Either learn both, or use some graphics library to provide an abstraction layer (like Ogre3D or IrrLicht). If you decide to learn both, I recommend not learning both at once, as you will just get confused. Instead, pick the one most applicable to your current circumstances (OpenGL? I think you said your friends are running OSX) and once you have that mastered, then you can consider learning the other one.