From running my XXX4 I found that a diff made the car better on really bumpy tracks, and to a certain extent helped indoors on some really tight tracks to as you have 4 wheel braking, just depended on how much you wanted the rear end to swing round.
Not had the chance to play with a diff in the front of my S4 yet, but I do know that it rotates round hairpins like nothing else with the one way in the front
