In order to fully understand the differences, you must understand both the history of the Schmidt-Rubin series of rifles and the way in which they work. The bolt face on a K31, K11, G11, or any Schmidt-Rubin does not rotate. Instead, there is an outer bolt sleeve upon which the locking lugs are mounted which rotates. The original G1889 rifle had the locking lugs positioned at the rear of the bolt and is the weakest of the Schmidt-Rubin series. Over time, the rifle was redesigned a number of times in an attempt to both shorten the action and move the locking lugs foreward in order to increase strength. The G11/K11 had its locking lugs located at the midpoint of the bolt and was the first rifle deemed strong enough for the GP11 cartridge. To my knowledge, no loaded ammunition is available for older rifles and thusly shooting them is strictly a handloading proposition. The K31 was developed out of a desire to standardize on one weapon rather than having both a long rifle and a carbine. By relocating the locking lugs to the front of the action, it was possible for the K31 to have roughly the same length barrel as the G11 while maintaining the same overall length as the K11. Also, the K31 is considered to be the strongest of the Schmidt-Rubin series due to its frontal locking lugs.
As a shooter, I would consider the K31 to be the best of the series as it will give roughly the same velocities as the G11 in a shorter, handier, stronger package. The older G11/K11, however, are still good shooters, can handle any commercially produced 7.5x55 ammo as well as surplus GP11, and are excellent rifles in their own right. As a collectible, the older G11/K11 is probably a better investment due to their scarcity as compared to the K31.
