This can be a sticky question. I am assuming that you have two sets of good tires, one of which happens to be new. Better tires go on the front if you ask me. You want optimal steering capabilities, especially in poor weather conditions.
There are those of the opinion that the worst tires go on the front. This allegedly allows you to alter your driving habit according the the steering feel. That is, if your tires are not handling very well do to wear, you will notice in the steering performance and have a tendency to slow down to compensate. Others will say that worst on rear will have a tendency to lose traction in the rear without the driver's knowledge and cause rear spin-out. Both camps have a valid point, and they both allude to putting the best on the rear. Some simply say that the drive wheels get the most rubber. If it were my car (AWD you say), my best tires would be on the front for best steering. Computer controlled anti-skid software that car probably has will help with the traction compensation.
Of course, the optimal solution is to always have 4 equally good tires on any car.
(Rated by 1 Council Member)