From my knowledge of things, salt water has a higher heat capacity than distilled water and a lower freezing point, HOWEVER, saltwater is a hell of a lot more corrosive and potential harmful to components... the reason I know this is because quenching a forged blade in saltwater often creates a better blade since the cooling occurs much quicker than in fresh/distilled water... but in the context of a pc, distilled water is the way to go.... *crazy fact*
I've always been using WaterWetter.
It's a high-performance car coolant used in race cars.
83% Distilled water
15% Car antifreeze, Propylene glycol (anti corosion and buffer)
2% algea killer
1 drop of soap (break surface tension)
Try to keep around a Ph of 8.
I want to use a Propylene Glycol for 2 reasons, you won't die if you or your pet gets to drink a little of it. It has anti corosive buffers and can keep the Ph at a good level for a while. And I can keep the radiator outside if I really need to.