I've owned or own and use all the models under discussion here. The post-war Contax is clearly superior to the pre-war Contax and Kiev which are not bad cameras at all (and can make good backup bodies if you own a IIa or IIIa). The IIa/IIIa is closer to a Leica M2 or M3 in terms of fit, finish and build quality and the prices generally reflect this. Of course the M2/M3 finders are better, but then again one still pays more for an M and Leica is a different universe in many other respects. Prewar Contax bodies in good repair are hard to find, while there are still many good working post-war bodies not in pressing need of a CLA.
A Contax IIa or IIIa in proper repair has a whisper quiet shutter, a silky smooth film advance, is light and easy to handle, and is a master work of finely finished metal and precision mechanical engineering. Need I say more???