Power Girl was mentored by Earth 2 Superman for years so she is more experienced, more tactical and a better leader (although she still has a temper). She obviously more mature since she is older (although New52 PeeGee is pretty dumb, Pre52 she was leading the JSA). And in the past she was drawn as much more muscular than the average superheroine.
Pre52 she was also one of very few characters left from Earth 2 after the Crisis. Although for about 20 years, her origin was that she was the time travelling granddaughter of an Atlantean mage sent forwards in time. That was retconned in the lead up to Infinite Crisis and she was once again made into the Kryptonian refugee from pre-crisis Earth 2.