I wonder - out of curiosity - about the per-capita civilian casualty rate in either of the two scenarios.

Well, the numbers I quoted counted both military and civilian casualties. Some of Roger's links above will probably give you a more detailed breakdown on the Japanese side. It's important to note that the distinction between civilians and soldiers is a bit tough to make when talking about the country being invaded, because if you come to invade my land, I'm going to grab a gun and sign up with the nearest military division. This is all a bit before my time, but my understanding of the Japanese fighting force is that there were a lot of conscripts who weren't really soldiers, but would have fought against any American invasion of Japan. Would you count them as civilians or soldiers?

Anyway as I mentioned above, the civilian death rate due to firebombing in places like Tokyo was just as high as that of the nuclear attacks. The only difference was the psychological impact of a brand new weapon with such massive power.