he have nothing at the moment, that's why he is undecided with KM or C. But talking about minimum focusing length, KM seems to be better. look at the widely used 80-200 lens. KM can hit 1.2m compared with 1.8m for C or N.
Vader is the one that teach me the importance of short focus distance. Some benefit would be the shallow depth of view and restriction of distance between photographer and subject. magnification would usually also increase as a result of the shorter focus distance.
If you are getting 3rd party lens for the reason of short focus distance, you do not need to consider whether KM or C or N as 3rd party manufacture lens for the different brands. vader, your ex 20-35 lens is actually very powerful. fast and quite also. but of coz with view factor 1.5, it does not meet your requirement.