so so so complicated! my general rule when baking is to follow the measurements of the recipe; if it measures everything in pounds/ounces or metric weights, use that. if it measures everything in cups/teaspoons, use those. it's so much easier that way i think.
what i know about baking/pastries and weighing is that the professional pastry chefs almost universally weigh their flour as opposed to measuring it in cups. this is because flour reacts so sensitively to the moisture content of the air. weighing it produces more even results in the pastries...