Well my first CRK was the Umnumzaan; but now I have three Large Sebenzas as well:
Regular(discontinued),
Insingo and
Classic Ebony inlay (same as the 21 ftmp)
My take:
It's true that the Sebenza is easier to disengage but the Umnumzaan is more ergonomic to the human hand. This is true for a variety of grips as well. The thumb fits nicely on the first groove of the spine for tasks like peeling an apple, and the forefinger fits on the second groove near the tip for detailed work. The handle itself is also more ergonomic fitting more naturally into the hand.
What does all this mean? Well imo If you are going to do hard cutting for extended periods I think the Umnum has a slight edge. If this will be more of a fondeling knife, that you will open and close hundreds of times a day, and you plan on using cheaper "beaters" for the big stuff, go with the Seb. In all reality, they are both great for both types of tasks and these differences are slight.
Styling is also a consideration with the Sebenza being the more classical of the two. If you like the more aggressive styling of the Umnum, it's a great choice. You really cant go wrong with either.