I really think that the small Seb and the Mnandi serve two different roles that complement each other. The small seb is a workhorse that can do just about everything and serve really well as an EDC.
The Mnandi is a true gentlemen's folder that seems more suited to the desk or for carry anywhere a fine writing instrument is carried, as in a shirt pocket or the inside pocket of a suitcoat.
Though undoubtedly the Mnandi is a strong knife, it is a thin design and just doesn't have the robustness or extreme-use ergonomics of the hell-for-stout Sebbie.
Since you already have a BG-42 small Sebbie, I wouldn't bother over getting an S30V as the functional performance difference has got to be very small. On the other hand, if your small Sebbie is the new style, then getting a small Classic Sebbie in S30V may be enough of a differentiator. Otherwise I would vote for the Mnandi as an excellent complement to what you already have. (There is a nice mammoth ivory and damascus Mnandi on Ebay right now!)