From the things that I have read:

Harvest clear - You get a clear, more able to think and function stone.
Harvest cloudy - You get a normal stone.
Harvest amber - You get a narcotic, couch lock stone.

The best think to do is to have a batch harvested clear, for tring to work or something while you are high, and a batch amber for when you really just want to watch TV, or maybe as a sleep aid. Or harvest some of your grow earlier than the rest. Experiment to see what you like.