The Mnandi is a beautiful knife and I recommend picking one up if you find a specific example you really like. By that I mean, get one with inlays that specifically call to you and if you can afford it, consider a Damascus blade. Many online purveyors show individual photos for each example they have, allowing you to pick out the exact knife you want.
I find the blade on the Mnandi to be capable of almost all the same tasks as the small Seb but the grip is not quite as comfortable nor secure.
One of the main situations in which I prefer the Mnandi to the Small Seb is that the Mnandi looks fantastic without a pocket clip and there's no indication that something's missing. I hate the appearance of a 21 with the pocket clip removed as it either looks like there's something missing, of if you put in the filler tab then it looks like there's some random part that serves no function, which ruins the utilitarian aesthetic of the knife, in my opinion.
If you are looking for a small knife that you can carry like a traditional and pluck open like a traditional, but that can be opened one-handed if necessary and locks solidly, then the Mnandi is the knife for you!
I also don't care for the polished scales that CRK uses for wood/ivory inlay Sebenzas so if wood/ivory is your goal, again this would be a reason to lean Mnandi from my perspective.
Personally, I have re-sold all of the Mnandi's that passed through my collection due to them filling a niche in between utilitarian and elegant that did not fit my tastes. I find that I would rather carry a sebenza for rough days and a custom slipjoint for relaxed days.