We'll give you a quick run-down of what the differences are, and why you might want one over the other.
The Chris King 2Nut is basically Chris King's high-precision take on the classic threaded headset. The bearing cap is threaded, and then there's a lock nut on top of that. The headset allows you to leave your steerer long by using spacers, a cable hanger, or just about anything else you can find to attach to a spacer (like a bell, for instance), and mechanically, it works exactly the way just about every other standard threaded headset does. If you have a threaded steerer that extends at least 5 full threads above the top of the adjusting (lower) cup, you can use the 2Nut, and depending on how much more than 5 full threads your steerer has, you can fill the space above the adjusting cup with spacers to take up the extra room underneath the lock nut.
If you have very little steerer tube extending above the upper bearing cup, or if you want to cut your steerer tube as short as possible for the lowest stack height, the GripNut is the best option because it uses a special expanding collet to clamp the steerer tube, which requires fewer threads.
So while it looks a lot like the 2Nut, it is functionally very different. The design is also more secure and resistant to going out of adjustment over time than a standard threaded headset design. The main downside of this design is that you have to cut your steerer tube to the correct length (between 11 and 14mm above the top of the bearing cup, not the adjuster cap). This obviously limits your ability to add anything like a cable hanger down the road, but on many bikesÃ¢â‚¬â€œespecially classic racing bikesÃ¢â‚¬â€œthis just isn't an issue, and keeping the headset's stack height low is a bigger priority.
Both headsets are available in 1", 1 1/8", and 1 1/4" sizes for straight steerer tubes, so you really can get whichever style works best for your bike and your intended use. If you've got any questions about these, or anything else we sell, don't hesitate to get in touch. We have experience with these on our bikes, and are always happy to help you decide which type will work best for you!