I recently had to make this decision and after much research I went with the 21.
I think the 25 should be completely thrown out as it is essentially an outdated Inkosi.
Reasons why I chose the 21:
1)Bushing Pivot
2) no second finger groove
3) Essentially the upgrades on the Inkosi either didn't appeal to me, or I didn't see why they were necessary.
Ceramic lock interface - I've seen multiple 21's with years and years of use and abuse and the lock has either not moved, or the movement has been extremely minimal. Lock Stick? In my mind a little lock stick is not a bad thing if your sebenza happens to come with a touch of it.
Thicker blade stock - Why? It's a folding knife
I would argue that despite the changes and updates to the Inkosi, the 21 is still just as difficult if not more difficult to produce because of the precision required for the pivot system.
As with all knife choices, I think it's a matter of personal preference. For me the "upgrades" to the inkosi didn't mean much. Not bashing the Inkosi at all. I'm sure it's a great knife.
I chose the timeless classic.