From my road and mountain biking experience the ball seems the best. But on a Coker, it does feel more stable on the arch. I have found myself switching between the two. I started this thread because a very good cokeur(say I'm not in the UK!) told me definitely the arch.
I ride on the ball of my foot. It's easier to make small adjustments to foot position and also let's you flex the foot up and down some...I think that's important at high revs so you don't get bounced off on bumps or if you made a mistake you don't get ejected off the unicycle.
-- GizmoDuck
The Uninam Tour 2008.....Hanoi to Saigon!!! www.uninam.net
The SINZ Unicycle Tour 2007....South Island, New Zealand www.sinzuni.org