I think OpenGl will be the best setting, but it might be using the default software OpenGL ICD as included with windows. In this case, what you need to do is go <A HREF="http://www.glsetup.com/" target="_new">here</A> and download GLSetup. This will analyse your hardware and download the appropriate ICD for you.
This should give you the best performance.
Software is the one you would use if you had a crappy graphics card. Direct 3D is good but I think OpenGL is better for professional work. It has been used for some time and is the tried and tested method.
<font color=red>"My name is Ozymandias, King of Kings:
Look on my works, ye Mighty, and dispair!"</font color=red>