OpenGL is a "Conventional" standards based standard, DirectX is a "standard" defined by Microsoft specifically for games.
So it takes about five years for something new to enter the formal standard for OpenGL and five minutes for something new to enter the DirectX standard. OpenGL has its origins in CAD programs and thus carries a fair amount of historical baggage.
However life is never simple so in order to allow companies to use features that are more relevant today the OpenGL standard allows companies to add extensions. nVidia has added a lot of extensions. nVidia also provides a lot of support to games companies such as Auran on how to get the best performance out of their cards so Auran are using nVidia code for example in the new texture compression of TRS2009.
In general you get better performance using DirectX but it does depend on the individual graphics card, motherboard and driver combination.
Cheerio John