OpenGL wil give a lot better effects because essentially the code has to be done for every platoform it runs on (i.e., assembler type). For instance, the shadowing and lighting in Doom III is so good because (a) they know what there doing and (b) all the stuff they are doing is calibrated for each machine so it might even be that the effects engine is faster i dunnno.

With DX, its not assembler but rather and API that, provided the hardware complies with the DX standard, all the effects can be done at an optimal level. This is why it is much easier to program in DX as opposed to OpenGL (at a developer level of course, where one has to consider multiple hardware platoforms).

Since all major hardware supports both OGL and DX and both run relatively smoothly and both give awesome effects, from a consumer perspective I dont think its feasible to compare them.