I think we can agree that KT266A and nForce 420-D are pretty much neck-and-neck. The nForce 420-D offers an integrated solution for those who want it. It also presents a viable option to OEMs, although I'm a bit disappointed with both of the nForce boards Tom looked at.
My thing is this . . . on my new rig I'll need a network card, a sound card, a RAID card, a PCI modem, and a FireWire card. Add in a slot cooler, and that maxes out most setups with six slots taken up. With an NForce board, I can ditch the NIC and SB Live, and have room to grow.