Umnumzaan wins in my book. Ill say the 21 in hand is a bit more comfortable, but that's it on the pros. Though the pivot bushing system is great, the take down and maintenance of the Zaan is easier. Not much tuning needed, the female pivot screw is open so any debris is a non-issue. Everything about this knife feels solid and consistent. Also I'm not for sure between the 21 and the Zaan which is thicker stock, but the Zaan actually has a finer tip without sacrificing strength. I believe this is due to the extra material from the harpoon tip. I also like the overall blade profile on the zaan better too, the belly isn't as drawn out. The deployment is different and tight, but I feel superior. You can the flick it, or draw it slowly.
The Umnumzaan is easily the best folding knife I've ever owned. All my Spydercos just sit at my desk. They get used around the house, but thats about it for now. Between my CQC-13, Superblue Endura, and my Zaan, I'm set.
I added the extra stand off and screws to mine, really tops it off. About the only time I post anymore is to praise the Zaan, i'm not sure if thats a benefit to my testimony, or a point against it.
If you wind up with one and hate it, shoot me a PM. Im always looking for a deal on another Umnumzaan.
Cheers!