Hmm... well to be honest. If you had to slow one or the other down lets look at who you'd be slowing down if you choose one or the other.

DirectX: Some Windows users that prefer DirectX.

OpenGL: Some Windows users that prefer OpenGL. All Linux, MacOSX and BSD users.

Now this isn't really taking into account Vista, but then again who cares about that now. It's not even out yet and I doubt that MS will get a away with slowing down another API just to boost their own for very long. [size=9px](Vista sales will suffer for it and there'll be more new Linux users in the world.)[/size]