best for spark transfer; dull or sharp edges?

The benefit of a U groove plug is that it lasts longer. The spark jumps across the gap where the resistance is least [ usually smallest gap ]. Every time the spark jumps, a minute amount of the metal is chipped off the electrode until the gap gets bigger; the spark then finds another spot to jump from with less resistance. U groove electrodes just provide more area, so last longer.

Plat, Irid plugs require less firing firing voltage because of the small volume of the electrode. The electrode could be steel, but would soon melt. That is why high melting point [ expensive ] electrodes such as Plat, Irid etc are used.

The smart hot rodder would see the benefit of reduced firing voltage as a means of increasing plug gap without getting misfires because the ign system is maxxed out. There is no such thing as too big a plug gap [ except for mechanical contact ].