Well it's all quite clear, if you aren't too lazy and you want CrossPlatform support and you don't want to be addicted to MS the OpenGL is your choice, if you only want to code for Win and don't care about the "Open" philosophy you may like DX more cause you have all that support from ... hmmm ... MS as i'm not a MS fan and tend to like other platforms too and i will probably never do any DX coding
OpenGL has more support from the community because it's a much larger community(Win, Linux, OSX, all *nix and let's not forget SGI themselvs) and OpenGL is a quite nice to work with.