Convex because this thread is about a technique to sharpen the blade. I haven't tried it (yet) but people seem to be enthusiastic about it. Basically you use a flexible abrasive surface. Anyone have some good Web descriptions? I've read Moras are traditionally sharpened this way.
I initially thought this thread was about hollow-ground (concave) vs flat-ground blades. I'm sure you know, but for the sake of the discussion, mention those are techniques for
making the blade rather than
sharpening it, each with its advantages/disadvantages.
Steve