Electrode shape in a triggered spark gap?
The electric field of a cone or point looks like this:



The electric field of a ring or disk simmilar to the one shown above looks like this:


Source: https://demonstrations.wolfram.com/ElectricFieldAroundAChargedRing/

The field through the ring is symmetric, when activated would most likely draw down the field uniformly through the axis of the spark gap and lead to a break down of voltage in the middle. And odds are that it would be best for the spark to not touch the ring. 

A point or cone off to one side would lower the electric field outside of the axis of the spark gap, and is not symmetric (if there is only one off to the side).