I prefer AA to AK. This is partially because the hand is already made. If there are only a few opponents then it may not need improvement in order to take the pot down. Also, you know you can move in any time you want if the pot gets big pre-flop.

AK has much more uncertainty. For example, if you move all in with it pre flop then you might be against a pair in which case it is probably a coin toss.