its debatable, because although you can get alot of money in pre-flop with AA, you'll either
1) win a small pot with AA or
2) lose a big pot with AA

However with AK, you will either
1) lose a small pot (your board didnt pair, so you dont have TPTK, easy fold, AA would be hard to fold even if it was obvious somebody had 2pair/tripz)
2) win a big pot (vs AQ, AJ, A10, KQ, etc)
*this stuff is taken from super system which i JUST read 4 times *

so its debatable as to which one is better... obviously if your facing an all in preflop, or a huge bet, AA is better... but after the flop.......

-Chris