In NL you can put a lot more money in the pot earlier on in a hand. AA is a great hand preflop and on the flop, but usually loses a little edge going into the turn a river. AKs is a good hand but is very board-dependant (a drawing hand) which usually needs all 5 board cards to flourish. So if you can get more money in at earlier stages of a hand, AA is the way to go by far.

-'rilla