I'm with the majority. Take your pick. The deer will never notice the difference. The 150 is probably moving slightly faster but the 165 more than likely has a bit heavier jacket for a more controlled expansion on heavier game like bear or elk and you will more than likely get better penetration with the 165 on a quartering shot. Broadside, both should pass through but the 150 will probably open up more causing a larger wound channel.