For hunting and recreational shooting, there is no real difference. Double action is superior for self defense (both for rapidity of fire and reloading), but I hope you would get a .357 Magnum Blackhawk for this, since the Single-Six is a .22. The single action, with a solid frame and a large pin running the full length of the cylinder, is stronger than a double-action, with its cutout for the cylinder yoke/crane and the cylinder supported only on each end. The GP100 will likely have a crisper and lighter SA pull than will a New Model Blackhawk/Single-Six, as well as a shorter hammer fall.