For me it is all function. This is not to say the 25 is lacking in any way, it is just not as good of a fit for MY preferences and uses.
Specifically I prefer:
1. Bushing pivot system. Because I feel it is what makes a Sebenza a Sebenza. That is a bit nebulous, but not too long ago, the Sebenza was about the only knife that had such a pivot. I am not sure who "invented" the idea or first used it on a folding knife...but it just seems like it was unique to the Sebenza before other makers adopted it (and the 25 does not use it...which I find odd (shrug)). The only way I can perhaps explain my feeling about this is like the integral lock...Chris invented it (others may use it, but Chris is credited with its invention by most other makers)...if there is ever a Sebenza that lacks it, I will not be interested (make sense?)
2. The deeper hollow grind on the 21 makes the knife a better fit to my needs and wants. Most of what I cut is fairly thin media, so a hollow(er) grind works better.
3. The blade width of the 21 is better suited to my needs and wants. Honestly, if I could get the dimensions of the large 21 with thinner blade stock (spine thickness), I would prefer that! This notion of thicker "hard use" blades on a folding knife is lost on me (just MY preference/opinion).
Hopefully it is evident that these are all preferences, and other people would be equally correct in concluding the 25 is more suited to their own preferences. That is why asking for opinions on what knife to buy can be flawed logic (because you are asking everyone else to answer the question only you can). Hopefully, you will gain some insight to make your own decision and not be persuaded off your own choice