I have converted a Contax Zeiss 28/f2.8 from the C/Y mount to the K. I like it very much. So much that a while ago (within the last year) I have seen two K28/f2 on the marketplace that I was not quick enough on either one. I actually started thinking about perhaps getting it, but what would I do with my CZ? I also have a converted Leica 28/f2.8, but I like the Zeiss much better (personal taste). I also have a Voightlander 20 (but its new glass - and the Zeiss 21 - either the Contax or the ZK are way too expensive), and the 31 Ltd, in addition to the K28/f3.5 Shift. As I posted before, in a moment of weakness (the price was good) a new ZK 25/f2.8 found its way to my doorstep. That's on top of the 12-24 and 16-45. I really do like the 20 to 30mm range. I have stopped acquiring - wanting to use them much more (to the point of what should I take along to shoot). So, that is perhaps another reason for being purposely too slow on the K28/2.
Personally, I would go for the K28/f2 - since its older, and with the old glass, something that is no longer being (or can be) produced. The ZK even though it is out of production, it is much easier to find. Also, the K can't be any larger than the ZK in size and weight. For all the nice features the ZK has (A lens, etc.), I do use the converted CK a lot more - probably based on size/weight. I have only read and seen good (actually wonderful) results out of the K lens.