I'd imagine that the one on top is safer, or at least it provides more ease of mind because the support is factory made and not from how a person rests the ladder against another structure, so the one on top definitely is good for making people feel sure that they are less likely to fall. The one on the bottom although appears to use less material, due to the one on top needing to have a second ladder attached to the main one. So the bottom one appears to be more efficient for it to scale up and to become large. I'd imagine that the one on top is best at smaller jobs at a persons home, but it can't be used by firemen or construction workers in which they need to climb several stories high. Overall, I would say that for society the bottom one probable is the best, although i think that the one on top has a specialty that increases the quality of life for many people, but overall i would say that the one on the bottom is a tried and true design and is the most efficient and effective design of the 2. But, out of any ladder, the pool ladder is the most effective, it is highly specialized, maneuverable, and held into place. No one will fall from it and it does a task that without it an entire pool would need to be redesigned.