I have multiples of the 25, 21, and the Ummumzaan, the one that I don't use is the 25.
I carried both the Umnumzaan and 21 for years (at different times) and never found a fault in either, I tried carrying a 25 and couldn't.
The 25's grooves force me into a grip where the blade edge is too far from my first finger so the edge goes to the Umnum there, I also feel the clip slightly when forced into that grip, as others have mentioned. The. 25 is also more difficult to open for me. The detent took a long time to break in but even after it did and after some adjusting of the pivot I find that I need to hold it in an awkward grip where the knife is too low in my hand to be "right". Neither the 21 or the Umnum have this problem- in fact it's hard to find a knife that opens better than either of those 2 designs, especially the Umnum.
I do prefer the shape of the 25's blade over the other 2 and like the high hollow grind, the rounded spine on both the 21 & 25 is a favorite feature of mine.
I constantly think of selling off my last 25 but everytime I pick it up I think that it's just such a nice knife that I don't do it, even though I don't use it. It "feels" more solid than any other knife I've ever had in hand, it's very nice- just not for me.
I've used a 21 to do everything a knife should ever be asked to do, don't be fooled into thinking it isn't a tough knife, it's a beast! The 25 may be tougher but I think that it's overkill past the point of a 21.
If you can check them out first it'd be in your best interest but if you can't don't worry about it, eventually you'll have them all
