Whey protein, Casein protein, soy protein... which one.... and when to have one?

is best for gaining muscle, not fat.... and is designed not just for body builders. i lift weights quite often and already am in shape but i want to gain some more deffinition without gaining fat or LOTS of extra weight. Help? when is the best time to have one and does it take the place of a meal or what? can you have one even if your not going to workout that day?