Mario gives a good explanation of how the three different settings affect the rotation of the front wheels here:
http://www.teamxray.com/xforum/viewtopic.php?t=1054
With a full one-way, you will have the most efficient use of power from your motor since both of the front wheels, which pull the car out of the corners, will always have 100% of the power as oppossed to a standard limited slip diff like you have in the rear. But since both front wheels turn at their own rate, steering is increased significantly and the car can become twitchy and oversteer a lot if the setup is off slightly. Brakes are non-existent, or very light if you turn down the braking EPA to about 50% on your transmitter so the rear end doesn't slide around when you use them.
With a locked one-way, the turn-in steering becomes more manageable but the trade off is that the front tires will fight each other and scrub more cornering speed throughout the entire corner. You still have the problem with brakes here as well. More stress will be applied to the driveshafts, cups and plastic blades as the front tires fight each other under power. Runtime is sacrificed some since the tires are fighting each other on-power.
With the multi-diff fully locked as a spool, you now have brakes! You can use them to stay on the gas longer and dive underneath to make a pass. Turn-in steering is decreased the most with this setting and you may have to change your setup to compensate. Runtime is sacrificed the most with the tires fighting each other on-power and the motor braking using battery power as well. But consistency is increased. Especially in low to medium grip situations. Usually good grippy tires (RP30's are wonderful), more rear droop, a higher rear ride height, layed down front shocks or more front toe-out will give you enough turn-in steering if you need it.