If it's all about getting started as fast as possible, I'd recommend OpenGL on a NVidia GPU. You can throw almost anything at it and it will display something. However, if you want to achieve decent performance with low programming effort, I'd recommend Direct3D. There are too many ways to do things in OpenGL, and beginners will most certainly discover the slow paths first since they are covered by many ancient tutorials (see NeHe). Additionally, debugging OpenGL apps is painful, although tools like gDEBugger can help.
