It was a combination of both. Harry Truman wasn't fond of the Soviets at all and wanted to demonstrate the power of the newly created atom bomb both to them and to the Japanese, assuming (correctly) that the Japanese would roll over and (incorrectly) that the Soviets would cower from American technological might.

The atom bomb also indirectly ended what would have been a DISASTROUS campaign on the Japanese mainland. Truman's decision to drop the bomb may have killed many people and affected millions, but it was hardly a "nuclear holocaust" (although that term is open to interpretation). Invading the Japanese home islands would have cost millions of Allied lives, and many, many more Japanese lives. Japanese citizens were being prepped to defend their home islands to the death. Plus, an invasion would take a much longer time to successfully achieve. That quote in the OP is a little sensationalized.