I always used to use D3D but the interface graphics and text are messed up in recent builds. OGL doesn't have these problems. Performance and image quality appear to be about the same with both.
The only issue with OGL at the moment is that the Launcher doesn't support all the custom resolutions that D3D does and resets it to 1024x768 if you change it manually in the registry. This isn't a big deal, since I can set all the flags in the Launcher once, set the resolution and then just open the game exe directly every time.
OpenGL's always been a bit slower for me on my 9800 than D3D, have there been any performance increases lately?
ATI cards are known to be generally faster in D3D than OGL. I think the latest drivers provide some gains in OGL though.