there are many differences between Dx and OpenGL. For developing with irrlicht, however, the differences really only matter much when you get to modding the engine. Some things are implemented somewhat better in Dx than OpenGL, but IrrlichtNX, the rest of the community, and the latest release have I think left few differences between irrlichts capability with the two rendering api's
You do a lot of programming? Really? I try to get some in, but the debugging keeps me pretty busy.