Comparing the specs of these two unicycles, I realize that I don’t know much about the significance of all the little (possibly large) differences between them. What is it that qualifies the KH20 to cost $600 while the Nimbus II comes in at $225?
The KH models are going to be higher quality for sure, but the critical question is to determine if the quality difference would be meaningful for you.
I will tell you that the Nimbus II is a fine machine, at least based on my experience with the 24" model. I rode mine at least 5 times per week, very often in off road conditions, for 18 months, and the only things that needed replacing were the pedals and tires. The stock pedals suck and should be replaced immediately. Although I have now replaced my nimbus II with a Nimbus Muni for off road, and a 29 for the street, the old II still rides just as well as ever.
The biggest differences are the frame, rim and cranks. The KH has an aluminum frame, and the Nimbus has a steel frame. The Nimbus rim is a 32mm freestyle rim, and the KH flatland has the KH Flatand rim, which is 47mm wide like a trials rim but 20" instead of 19". This gives more stability and bounce. Finally, the KH comes with KH Moment cranks, which are indestructable and better for flatland than the QU-AX cranks on the Nimbus. Also, the KH has a higher volume tire and Twisted PC Pedals.
Both unicycles are high quality, but the KH has a few lighter/stronger components and is aimed at flatland unicyclists, whereas the Nimbus is aimed at freestyle unicyclists.
I would add that the wider, stronger rim is less likely to cause the tire to fold over or come off the rim in large side hops. Unlikely to happen in flatland IMO, maybe in a scetchy landing on a spinning trick. Also you can run the tire at a slightly lower pressure w/o pinch flats (how much?, I’d say </= 5 psi).