You'll have to make the decision based on what your food stocks look like, I think... and depending on how many mouths you have to feed. At Normal and Easy, I don't have too much difficulty getting science first - and then getting the food is a breeze...in fact, most times I get science 3 before food 3... but you really have to make a determination based on what the state of your game is for the Hard version... don't think they're any blanket answers