[When I first bought the 180, I was dissappointed with the lack of "3D effect" (ie. color separation) that I've grown accustomed to with Contax Zeiss Lenses. But, when I compared it with my Nik*or 180, the accurate color reproduction was obvious.
The choice between the 180 and 200 is determined by your needs. Contrary to what others say, the extra 1 stop and shallow depth of field do make a difference. When you push the limits and alreay use ASA400 film, you need a 2.8 lens and not ASA 800 film. ie. faster lens compared to faster film.
The 180 is good for portraits, and I use it to photograph my kids at the playground, which is shaded. The ability to take candid portraits (head shots), and soft bokeh combine to give a very nice effect
I've only used a borrowed 200/4 for a few minutes, and the images appear sharper. Is it because of more DOF, I cannot remember (anyway, my comments are all subjective). I would recommend the 200 for taking photographs from a distance (maybe sports or landscape), But for portraits, the 180 is just more pleasing to the eye. As others have mentioned, if you don't mind an f4 lens, the 100-300 is a good choice. The MTFs and convenience of zoom, speak for themselves.
I do use my 180 for lanscapes, but I stop it down to 5.6-8. If you plan to do that, no point paying for the extra 1 stop.
I haven't mentioned weight and size because that's never been a consideration when I buy lenses. But yes, the 180 is heavy and is not comfortable to use when I put it on my backup Yashica FX103 body.
Lastly, there was a quote from a Zeiss employee comparing these 2 lenses. His personal choice was the 180 for 2 reasons: the extra 1 stop, and the ability to use the Mutar 2 (the 200 can only use the Mutar 1). I think I downloaded the document from Contaxinfo.com
Hope this helps. ]