Depends if you want it for racing or endurance work,
Single wheel for sprinting to the best peg, tighter turning circle as well means you can oupace anyone with two wheels.
Two wheels for distance work, better balance means less stress on the arms though they turn like an oil tanker, better over rough terrain as well, any racing snake with a single wheeled GTI barrow will lose it come the first mole hill, meaning the best peg will be yours.