I play lots of "real" disco too, so I understand what you mean about needing a smooth response when you drag your finger on the edge of the platter. So that was you on the Denon forum complaining about it? I saw that the main Denon guy answered you (I think) saying they had fixed it for the 3900, but of course he would say that!
The only way to know for sure is by actually having a go - that's why I was asking on here, as I thought someone here might have tried one. I might pop into town on Saturday and see if I can find one in a shop to try. I think darrylfunk has used them before and likes them, hopefully he can chime in with more detail about the platter.
I know that the ns7/v7 platter is supposed to feel good, but I don't think it integrates with Traktor that well (it's designed for itch) and it doesn't have the facility for CDs or USB sticks either (which I really like about the Denon). I also think the Numark looks a bit like a toy, while the Denon looks like a piece of pro equipment (especially since they removed those silly effects).