I hear a clack when mine engages. But it is rather soft. Not to be mistaken for the click made by the front auto locking hubs when changing direction while in 4x4 (if the hubs are not manually locked).
I have also heard of others saying that they do not hear theirs clack at all.
The ARB air lockers is probably the safe route, given the wide spread use and therefore support you will get for it.
But I have also heard of the magnetically engaged lockers, which I think is a better technology, but not common in SA.
Standard vs Air locker:
The standard locker if used correctly is pretty good. Where it has broken, this typically happens when driving off thinking it had already engaged, when it has not done so yet.
Once engaged it will not break unless you try too hard and abuse the vehicle in an obstacle (my view).
It has, as Peter says, only a few large teeth.
I understand that the ARB air lockers has more teeth and is stronger. But it is not full proof since it still relies on an air tight pipe from the compressor to the diff, to engage.
In an obstacle, if only one wheel has traction, and you are stuck, using too much power while the diff is locked, can result in serious damage elsewhere in the drive train.
Therefore, I suspect, if you have a choice, it would be better for the the standard locker to break rather than the side shaft to bend of the prop shaft to break off.
Having a super strong diff locker is therefore not necessarily such a good thing...
The weakest link in your drivetrain should be one that can break, allowing you still to drive home.
As far as I know Thabbies and Offroadbiker on this forum have front lockers.