The inkosi and the 31 are fairly similar. They both use the ceramic ball lockup: a hardened ceramic ball in the end of the lockbar serves as both the detent ball and "lockbar insert". Makes for a very smooth lockup with no lockstick. The ceramic ball is EXTREMELY hard. It won't wear out in your lifetime
Differences: the inkosi has thicker blade stock.
The inkosi has the grooves in the handle. Alot of people dislike them, but they work great for me. The fit perfect.
The inkosi has an angled pocketclip. Another nice feature as it prevents pressure on the lockbar. The sebenzas clip runs straight along the lockbar
The inkosi has a thicker pivot as well.
One thing worth noting is the 31 lacks the famous pivot bushing everyone talks about. This is found in the sebenza 21. It allows the knife to have the same action, regardless of how tight/loose the screws are tightened. It also makes locktite not needed.
With the inkosi and the 31, purple loctite is recommended(and supplied in the box, with lubricant and assembly tools)
If you do go with the 21, also note it does not have the ceramic ball in the lockbar thingy. The end of the titanium lockbar is hardened. I haven't heard of any problems with it.
Regardless of what.you choose, if you like what you get, you'll want to get the other. You will never feel satiated
Hope you have a decent paycheck friend