Hi, thats a truly hard choice to make.
i fondled the 25 at Blade and it was sweet in the hand, i can't wait to get one when the production models start shipping with regular prices. its a great size and seemed to be the regular CRK sturdy that you've come to expect from your small sebbie. is it the perfect knife, probably not, but even using it daily for an entire lifetime I'm sure that ill never wear it out. my main gripe with it is the lock using the same lock as the Umnumzaan, it just bugs me that it only rests on that small ceramic bearing instead of the entire face of the lock bar like it does on the Sebenza's. is this a problem? not even close! I've been carrying a Tanto Umnum for over a year and a half and have been none to kind to it, it's lock hasn't worn in at all and is still as strong as the day that my Wife gave it to me(the knife). its proven plenty strong through some very hard cutting and has become such a favored tool of mine that the only time that its out of my pocket other than when its being used is when i feel that a smaller knife is right for the day(5 or 6 times a year), which is when i carry Mr Reeve's other perfect creation-the small Insingo.
i only fondled the 25 for a little while but i can say that if the production model is as good as the first 25 are, and we all know that they will be, you really can't go wrong with either knife.
ill give the typical internet answer, if you can afford it, buy both!
i suggest getting the Umnum first since its available second hand for a decent deal almost daily, then after the hype wears down and people start to have stock or move their 25's jump on one for a few bucks off of retail.
happy shopping to ya!