After a few early generations of 'experiments', I would say both KT600 as well as nForce2 Ultra 400 should be equally mature. (I personally consider KT600 as a KT400B ...) In terms of 'real life' speed, they are pretty much equal (assuming you are NOT interested in the onboard 3D).
Therefore, it is very hard to pick based on technical aspects. Because of that, I would pick nForce2 solely based on political reason - VIA now has both K7 & P4 licenses while nVidia still has only K7. I suppose nVidia can be a bit more focus and not strong-armed by Intel.
Of course, this is just me.