That depends on what you're using your car for mainly. One thing to keep in mind is, The tighter your axles are sprung, the better traction you have in the straights, but increased tire wear everywhere else. This is desireable if you're drag racing. Not recommended for the street.

1) OEM uses clutches to "lock-up" the axles. Its the least tightly sprung and doesn't last as long as an Auburn "Cone Type"

2) Auburn, uses "Cones" to lock up the axles. Its sprung up tighter than the OEM and lasts longer. Its drawback is, its NON-REBUILDABLE. Although, Auburn has an exchange program where they'll take your worn unit and give you credit towards a new one.

With the above 2 units you cant go wrong, especially on the street.

I haven't tried using a Locking unit.

Steer clear of using a spool on the street. Tire wear is horrible, wheels "Chirp" and Hop while cornering, and if your tires are really wide or sticky, make sure you have a spare set of axles.