Any critical analysis of the situation presented to President Truman clearly shows that dropping those two bombs was the more humane decision and that is only considering Japanese casualties. Here is the Cliff Notes...
http://en.wikipedia.org/wiki/Operation_Downfall
Operation Downfall...the invasion of Japan...
"Casualty predictions varied widely but were extremely high for both sides: depending on the degree to which Japanese civilians resisted the invasion, estimates ran into the millions for Allied casualties,[1] and tens of millions for Japanese casualties."
The Japanese lost 66,000 dead and missing as military casualties on Okinawa as well as between 75,000-140,000 civilian casualties.