Just wanted to know what the difference is for us.

I know that when you buy coloured gunge it uses powder for easy postage but Iv always used poster paint you can get from supermarkets?

What's the best?

GMR

Guess it just depends on the situation. As you say - often cheaper to ship powder and add essentially 'free' water on on arrival. Though sometimes you can only get pre-mixed materials. You also save a bit of time and labor by using pre-mixed. Though, you can generally only make the consistency 'more liquid' with pre-mixed materials. If you buy powder, you can get most any consistency.

Probably a dozen other advantages / disadvantages to each, so overall, just go with what makes you happy!