Lower numbers for dynamic elongation are desirable if you wish to prevent a large fall. However, a longer fall means a softer catch since rope stretch slows you down before reaching the bottom of the fall. This also means the climber will experience less impact force. The lower the number is for impact force, the more comfortable a fall will be. Therefore, although it may seem like a good idea to buy a rope with as little dynamic elongation as possible, in reality, more dynamic elongation makes for safer makes for safer climbing.