I believe I read that you should have at least 800 pounds of energy to effectively take down a whitetail. At 200 yards the 250 in the chart above just meets that, while the 300 still has 1000 pounds of energy.
I shoot 300 grain Parker Productions Ballistic Extremes and they wallop deer - bang flop. But never got a chance to shoot one over 85 yards.
The 300s are also longer than the 250s so you should have a better ballistic coefficient with a 300.