נєѕѕι¢α (ανα'ѕ мσмму)
The only difference I know of is that the ready to use ones have preservatives that prolong the shelf life, which makes my daughter sick, and that the ready to use is thicker. Other then that I don't know. I use Similac though. I looked it up and heres what I found: "The biggest differences between powder, ready-to-use liquid, and concentrated liquids are their cost and the way they're prepared. Some parents don't mind paying a little extra for the convenience of prefilled Enfamil® Nursette® bottles. Other parents prefer to buy the more economical powdered formula and mix it themselves."

kimchi
i don't believe there's much of a difference except for the "convenience" factor of not mixing formula. the powder formula can tend to be a little "chalky" if you use too much powder and not enough water.... but i personally don't like the ready to use liquid since it needs to be refridgerated and then the formula needs to be warmed ... i'd rather quickly mix up a batch.