From the mouths of noobs... but this is how I understand it...
Water has the ability to cool/freeze the structure very quickly, but is hampered by the vapor release and inconsistency, it also might give excessive hardness, resulting in an increased risk of a broken or warped blade.
Brine decreases the formation of the vapor blanket, so it could reduce the risk of warpage, but speeds cooling, so could still result in more cracks than oil.
Oil at the same temp as water will cool the blade 44% as fast (
http://www.navaching.com/forge/quenchants.html), resulting in somewhat less (but more consistant) hardness and a lower risk of deformation and cracking.
The temp of the quench also changes the result, water become less effective when the temperature gets higher (my guess- because the vapor layer generates faster the closer you get to boiling). Oil based quenches become more effective at higher temps (I expect the lower viscosity allows more heat transfer.
The other variable is the steel. Is it O-1, w-7, a-3, h-13... I've been told that only the W steels do well in water quenches, but that they do well in oil as do O steels.
I've read a few books on knife making as well as DVD's and they all recommend oil or transmission fluid for 5160 or 52100.
Of course, my beliefs will be worth more when I finish my first real knives rather than blacksmith tools...
