I am not expert at this design level since designing ICs likely follows more rules than designing ordinary circuits, in order to let the integration be practical as specifications and economical for production.
So I think, a professional CAD program for ICs (say digital) gives the designer the end specifications (of the inputs, outputs... etc) and the estimated cost of production of what he is designing. This helps him do some tradeoffs.
While Fang and Cheng circuits have the same number of transistors (PMOS and NMOS), they surely differ in some characteristics. For example I won't be surprised if they have different propagation delays for some (if not all) possible transitions. So choosing which one is better depends on the other parts of the IC.
About NAND vs AND and NOR vs OR, I think, though I may be wrong, that naturally building NAND or NOR with the active elements used in the ICs seems to be easier than directly making AND or OR. But you can always check to how far this could be true.
Kerim