Couple of points:
-Japan's land army was in decent shape, most of their loses was in naviation and navy
-there are 4 or 5 spots where invasion similar to Normandy would be possible, and they're all much more defensible.
-firebombing of Tokyo had more dead than both atomic bombs combined, conventional invasion would have inflicted staggering casualties on japanese people
-fanaticism of japanese troops, banzai charges are very well documented, even their wounded used hand grenades to take as much enemies as possible
-America wanted unconditional surrender of Imperial Japan, Supreme Council would continue to wage war as long as it hoped to force something else
-War was ended without additional allied casualties, huge impact on Truman's decision
There are lot more similar points for both sides of argument, just one thing; both Nagasaki and Hiroshima were targeted because they were average cities, not too small, not too big, with military presence to justify using the bomb, there were quite a few other candidates. In the end, there is no right decision, war had to be won, and price would be big in any case.