Pressed and loose powder are exactly the same product; pressed powder is loose powder that has been compressed into a compact. Loose powder is more economical and allows for more complete coverage because it is fluffier; it adheres more easily onto a brush or puff.
Use loose powder whenever you are at home. Use pressed powder for touch-ups during the day and whenever you travel, since loose powder is too messy to carry with you. (See chapter 11.)