the hydratrack i know very little about, something to do with the way the lsd locks up and releases with its viscous design....i have heard they're kinda **** for the cost...
the KAAZ on the other hand i cant recommend highly enough...bloody great lsd's! they're basically the lsd of choice for performance applications and are so damn reliable and strong. unlike other lsd's which work with a cone and seat design or similar, with torque forcing them to lock together creating the lock between both wheels, the KAAZ has heaps of little clutch packs.
they're very tight and are pretty much locked most of the time, only having the slightest give in them, at low speed turning you can just hear the packs "groaning" or "chattering" as they slip a bit to allow independent movement of each rear wheel. as soon as torque is applied they force together more and more, creating essentially a locked diff similar to what you'd get with a mini spool etc...
i always get comments about how predictible and tight my KAAZ is, i absolutely love the damn thing. they're expensive, but well worth it if your after a good lsd and dont want to risk it with a spool. another good thing about them is you can get 2-way KAAZ lsd's....meaning your diff is lock under deceleration as well, which is great for high speed cornering.