Leica didn't catch up with Contax until the M3 was released 20 years after their II/III. The M3 surpassed Zeiss in certain respects, in a few respects (Eg rangefinder accuracy) Leica have never surpassed Contax. You should bear in mind the very strong pro-Leica stance of most forum members here, objectively Contax hosed the Barnack Leicas in usability and optical performance. Why do you think Amedeo Contax adapters for Leica are so popular?
By all means get whichever camera you find easiest to use, it's a personal preference of course. Perhaps unusually, I came to top flight rangefinders via Contax, first. Lately I've been using a IIIg a bit and, yes, it's a lovely little camera but my Contax IIs kick its butt for ease of use (the separate window for focusing is small and pokey and a PITA). Separate dials for slow speeds. Rotating shutter dial. Loading one's a joke.
There are good reasons why the M3 fixed all these issues.
I'd take a II over a IIa personally, but a IIa over any screw mount Leica unless I had to fix it. I don't mind working on IIs, the IIa has some retrograde features...