"Here's the more prosaic question: I have a 180mm K/L and am considering either acquiring the 250mm to complement it, or opting for the 210mm and disposing of the 180mm. I hoped to elicit views on the relative merits of each of the two lenses in question."
Now I understand what the question is.
I don't know about the 210. But I can't imagine why you would get rid of your gorgeous 180mm K/L for something that is only about 15% longer.
I can talk a little bit about my brief experience with the 250, though. I have a 250mm f/4.5 plain (non-C) lens that is broken, so it can only be used wide open. I have used it a few times on transparency film just to see what it looks like. It is fine by me with an 8x loupe (which is greater magnification than I would likely need for a print from this camera, as most of my pix are 11x14 or smaller), but then again, I tend to love uncoated or single-coated glass for many things; and certainly the things I will mostly shoot with a 250mm lens. It is not the sharpest medium format lens I have seen. However it is still sharp, and definitely gives me nothing to complain about. I am sure it is even sharper stopped down, and maybe the newer versions are better still.
I would not get too stuck on the idea of judging a lens based on specs and whether people on the Internet think things are "dogs" or not. First of all, no lens that can be used is a "dog", and any RB/RZ 250 can definitely be, and have been, used for large numbers of great pix. Second of all, on the Internet, you have to wade through vast pools of sewage to find select gold nuggets, so you can often do yourself more harm than good by asking for something. What on Earth did people do before the C, APO, K/L, whatever the hell they're called now, came out? They shot with what was there, made a living at it, and made some classic pix that we all know. I would start with a cheap model to see if you even like the focal length. If you want a more modern one after a while, start keeping your eyes peeled. When you find one, pick it up and sell the old one. No need to rush out and drop all the dough on the fancy-ass models if you don't really need them...and I would say that nobody really "needs" them.