I have the first Mantra only, and I recently also got a Sliverax. Full disclosure: I haven't done more than fiddle with the Sliverax because I sent it into spyderco because the detente on the one I received was so weak as to be unsafe, imo. Apparantly this is a common complaint on the model. However, everything else about the Sliverax was amazing - the cool design, the compression lock on a flipper, the super "locked in" feel in hand, the light weight. I really hope they'll fix the detente for me or replace the knife.
I also really like my Mantra, it's basically like a non-frn delica with a flipper, which is awesome imo. A version of that with carbon fiber and a comp lock sounds even better on paper, although I don't like how they changed the blade shape to accomodate the comp lock, and tucked away the flipper tab into the handle when the knife is open. The more exposed tab on the original makes me feel like I won't slip up onto the blade. So the new one does not strike me as a "confident handguard design".
The Sliverax is also a larger knife that doesn't feel like a larger knife. So, if I had to pick one, I'd definitely go Sliverax, and cross my fingers for a stronger detente than on the one I got.
"Be excellent to each other." - Bill S. Preston, Esq.