I have to agree with Will on this. In my experience, I learned most capabilities of Direct3D (pre-shader era) in two years, while it took me only a week to learn most of OpenGL (in fact, there once was OpenGL-based version of Asphyre which we didn't publish ).

If I would start a 3D game development and would have no "framework" at my hands, I'd definitely go with OpenGL. The reality is different, since I have Asphyre to back me up now (it provides limited high-level 3D support already), so Direct3D is the only way for me now. However, your case is different and I think you'll have more luck if you go for OpenGL (you can use GLScene to get started).