[Myrddin]

Two designs were deployed that day. An implosion weapon and a gun assembled device. The implosion weapon was the design that was uncertain. It required less material, but much higher precision. The gun assembled device required more material, but is a sure fire design. The Little Boy gun assembled was used first on Hiroshima. The Fat Man implosion device was dropped on Nagasaki. It had a higher yield with less material, but the target/deployment did less damage due to terrain effects.

[whd23]

The implosion weapon was the design that was uncertain. It required less material, but much higher precision. The gun assembled device required more material, but is a sure fire design.

Correct. Also important is that "Little Boy" used enriched uranium while the "Fat Man" was a plutonium bomb.