Well, if your client wants 3D to run in a web browser, OpenGL is not an option. Web GL might be, but its not very widely supported at the moment. Flash Molehill is intended to be supported widely, on as many browsers as possible, as well as in Air (standalone outside the browser), on multiple platforms (PC/Mac/etc).
Other than that, the functionality available via Molehill is similar to that of OpenGL-ES. Broomstick sits on top of Molehill and provides a scene graph and other a suite of typical 3D engine features. I hope that answers your question. -Dave
