Either would do fine at 3-400. I would choose the 6.5 because I really put an importance on seeing impacts on animals. In my rifle arsenal, bigger the caliber and cartridge, heavier the rifle and bigger the brake. I want to be able to see impacts at the distance I plan on shooting the rifle at. Breaking a shot, to then be frantically trying to get back on the animal to see where it went, where you hit, is not a great feeling.