I did an informal comparison of these two scanners when I bought the Nikon to replace the KM. I had not been happy with the KM's reproduction of color balance and with its lack of ICE. Restoring original color balance was a real headache for me w/ this machine. The Nikon's ability to render colors accurately has really won me over, and ICE is great for removing scratches and dust.
While I think the Nikon is a better machine overall, I think that if you are only scanning B&W, for which ICE is useless, then there is a logic for getting the SD-IV. In the comparisons I conducted (again, very informal on only a 6-8 samples), I found that the SD-IV has great ability to reproduce highlight detail without blow-out. The Nikon's lamp seems fairly harsh, and can exaggerate grain. This is something that is controllable through proper exposure and development, but the exaggeration of grain in B&W is something to which most scanners are susceptible. I have read that the SD-IV's lamp is more diffuse, and thus produces more agreeable grain and a smoother scan. My tests corroborate this claim. I cannot answer your question re: dynamic range, but I can say that the SD-IV's dynamic range was adequate for my needs.
I think a more important question here wrt the KM is what are you going when it breaks, since it is no longer being manufactured? In this regard, I have found the Nikon rock-solid, it is a plug and play machine. My KM, in comparison, gave me some troubles and went in for warranty work at one point. Later, I thought it was broken, but it was only a software hiccup.
