[Azred]

Part of the psychological terror caused by nuclear weapons is that not many people are still alive who have witnessed/experienced such devices firsthand. The rest of us can only imagine what being on the receiving end of a nuclear attack would be like; it is like asking "what would you do if you were the last person alive?"--you can't answer truthfully because you just don't know.
To answer your original question--neither. There is no difference between destroying force "a" with either nuclear or conventional weapons, because dead is dead. Conventional weapons also produce lots of atmospheric dust, especially if weapons like BLU-82B (the most powerful conventional explosive made) are used. As is being reported recently, there are new concerns about packing radioactive material into conventional bombs to spread the "fallout"--this would achieve the same effect as a nuclear bomb.
Talthyr Malkaviel is right in that the initial designers/users of atomic bombs didn't know what their true effects would be; the only test was in the middle of nowhere; some thought the atmosphere would catch on fire! Follow jtqbe's advice--read "The Making of the Atomic Bomb"; it's been a few years (so I need to read it again) but it is an extremely good book (beware of the pictures in the middle).
Nuclear weapons are the ultimate form of the ancient practice of "poisoning the well", in which you would make an area uninhabitable to your enemies if you couldn't have it, either. Now that is spite!