if the number jump is about 10 or 20ish, like from UN52 to UN72, or UN40 to UN51, it usually means better quality, but a UN53 is not really any better than a UN51, it's just a newer model. In fact, some would say that a UN70 is better than a UN73 because the UN73 is cheaper and heavier, with a steel right cup, and the UN70 is lighter with an alu cup.