best for spark transfer; dull or sharp edges?

RRR,
[ Post 12 ]. Talk about the electrode material can be misleading. The copper core is NEVER exposed to the flame. It is a cartridge buried inside the centre electrode, usually a composite of steel & nickel.
The info I posted can be found on the net from releases by NGK, but I also have some technical releases from NGK, which is where the above came from, #T01-7.
Too long to type out all of it, here is a snippet: 'With fine wire electrodes, the electric field is concentrated across a smaller surface area with sparking able to occur at a lower voltage.....' A graph included shows a reduction from 24kv firing voltage down to 20kv going from the standard 'thick' electrode to a 0.6mm electrode, for a 0.060" plug gap.
Also: 'Reducing the centre electrode diam of a spark plug reduces what is commonly referred to as the quenching effect [ ie, heat energy loss from the hot flame kernel to cooler electrodes as the flame attempts to expand & combust the mixture]'.

The reduction of the resistance of the short length of copper [ vs steel ] in the centre electrode would be immaterial in spark delivery because there are much higher resistances already in the circuit [ coil sec res, leads, rotor gap etc ].