for motorkhana and the choice of anopen or locked diff - definitely locked. even for bitumen motorkhanas. open diffs are only good for going to the shops.

an open diff will be useless for trying to flick the car around tight cones. it will also be useless trying to get traction and drive through slaloms, and out of any corner or away from each witches hat, or when doing j-turns etc etc... but you will need to play around with the suspension settings to get rid of the understeer, and get some decent tyres for the front.

A good handbrake is also VERY important for motorkhanas.