The difference between the two headsets is that the GripNut design (while quite ingenious) does not allow the use of spacers or front brake cable hangers (generally found on Cyclocross Bikes) which means that your steerer needs to be the proper length for the headset to remain properly adjusted, you might need to cut some of the steerer off if your current headset needs spacers to make it work, or you might not since King headsets have a pretty high stack height. The 2Nut comes in where the GripNut leaves off allowing a more conventional adjusting mechanism which DOES allow for spacers or brake cable hangers to be used. Besides that they are essentially both the same incredibly high quality headsets from the best manufacturer in the Biz.

-Ryan