The question is a vexing one at first glance. The facts are, the 308 is about 100 f/s slower than the 30-06 with a 150 grain bullet at max loads. A 30 caliber bullet just doesn't hit harder because it was fired from a different rifle or out of a different chambering, they hit harder because the velocity is greater. Start the same bullet in a 30-06 at the same velocity as a 308, there is absolutely no difference in performance at any range. One might make an argument the 308 is an inherently more accurate round than the 30-06, but in a hunting rifle, I doubt most people can discern the difference. The same placed shot with either rifle at your stated distance will cause the same results on game. Dead is dead, wounded is wounded. As to a starting velocity difference of 100 f/s making a determination in how far you can shoot, if you are a sophisticated enough shooter to shoot 500 yards (there are few that can do it), you are also sophisticated enough that you know the difference in drop in both the 308 and 30-06. Bottom line, either caliber is fine. It's a matter of personal preference.