Setting the ISO in camera would be the preferred option because you will capture more detail in the scene if you do that. See article click here. You would not be able to recreate detail not captured in the first place by adjustments in PS or other software.

And every one said, 'If we only live,
We too will go to sea in a Sieve,---
To the hills of the Chankly Bore!'
Far and few, far and few, Are the lands where the Jumblies live;
Their heads are green, and their hands are blue, And they went to sea in a Sieve.

Edward Lear
http://www.nonsenselit.org/Lear/ns/jumblies.html