under or over diff exhaust.

[Ivan141]

It's only adviseable when your car is lowered a lot, so there is enough clearance under the axle for it not to hit the exhaust.
The advantage of an under-diff exhaust is that is has less bends. Not something worth bothering about unless you have a turbo engine or some high-revving tuned engine. But then again I come from the country that has speedbumps on every street, which makes it even less desireable to reduce the ground clearance.