That's a false dichotomy. They weren't going to nuke us...
No, but if they'd had the chance, they would have used their bioweapons program to decimate our civilians. The postwar evidence of their weapons research showed their programs were very advanced. Their testing of bioweapons on the Chinese inflicted up to a million dead. And their tests on American POWs included live dissection to chart the progress of fatal diseases.
A History Of Biological Warfare [1932-1942: THE ORIGINS OF BIOLOGICAL WARFARE / UNIT 731]
"Book Examines Japanese WW2 Biowar Program" By Aleksandrs Rozens
So let's not pretend that their failure to hit us was anything other than we didn't give them a good enough chance to succeed.
You might read this recent thread on some of the latest information released due to declassification of source documents. What it shows is that Japan was not ready to surrender and that our military was not really ready to proceed with their invasion.
Why Truman dropped the Bomb (Long but a very interesting read)
Given that about 200,000 Asians were dying per month due to prolongation of the war, even a delay of a single month in ending the war would have resulted in twice as many deaths as did Hiroshima and Nagasaki combined. It was in everyone's interest, including the Japanese, to end the war as quickly as possible with overwhelming force. Other choices such as what you appear to imagine would have actually resulted in far more deaths for both countries and the conquered Asian countries.
You really need to acquaint yourself with the latest info. Your views on the subject are unsustainable in the face of the newly declassified evidence.