Hi Robyn. The key ‘word’ in hydration is “drink.” So the real answer is whichever of these is more palatable to the person involved so that they will drink more. Personally for me it’s cold water.
I hope that this helps.
Room temperature water is more readily available to the body for hydration, and it takes less energy and time for the body to use.
For faster absorption/general hydration, room temp is the better bet.
As far as performance during exercise goes, I’ve read studies saying it helps(mostly in relation to keeping down the body’s core temperature), and some saying it hurts(mostly in relation to strength related exercise, like the bench press – but this was only in one particular study, to my knowledge).
Most studies I’ve read indicate that cold is better for performance.