Assuming you are running a stall variant, Leftovers are the better choice. Leftovers allows you to continuously regain health even when using King's Shield. Sitrus Berry only restores 1/4th of your health whereas Leftovers restore 1/16th every turn. If Aegislash lasts four turns then it will have regained the same amount of HP with Leftovers as it would have with a Sitrus Berry.
In summation; there's almost no reason to run Sitrus Berry on Aegislash.
