A lot depends of how the motherboard maker uses the chips, not just the chips themselves. If it was all up to the chip maker than all boards would overclock the same, setup the same, etc...
But I am leaning toward the KT400. The nForce2 should put some heat on the KT400, but the KT400 should be a little faster, BUT the nForce2 should have better sound(for the most part, again see boardmaker) and the nForce2 will have the option of on-board video.
VIA for me, but a little competition never hurt (see prices of CPU's after AMD put a run on Intel)