In RWD drift car, you want LSD effect under accel and decel, hence it's called a 2-way diff, because it provides LSD effect i both directions, accel and decel. Equivalent setting in GT5: Initial 10, Accel 60, Decel 60
Some race RWD's use what is called a 1.5-way, which provides full LSD effect under acceleration, but only 50% of that LSD effect under braking, which helps stop the back end braking away on the way into corners in tail-happy cars. Equivalent setting in GT5: Initial 10, Accel 60, Decel 30
FWD's use what is called a 1-way diff which only works under acceleration, meaning that turn-in is not affected in any way. Equivalent setting in GT5: Initial 10, Accel 60, Decel 5
Most competition drifters use 2-way diffs as they provide the added advantage over locked diffs of being able to be 'unlocked' simply by dipping the clutch. In a welded diff, there is no way to open the diff up to give the back end grip. LOTS of people use welded diffs for practice/fun, but for comps they have SERIOUS limitations.