Regarding the thread's dichotomy, an invasion would've been more preferable.
It would have led to far more deaths, and a prolonged military engagement. How is that preferable?
Deaths of combatants or civilians? I find it seriously hard to justify the murder of civilians.
It would have led to more overall deaths; I don't know the exact breakdown.
There's a distinction to be made, though, between those who choose to fight, and those caught in the middle, like the residents of Nagasaki and Hiroshima. If war is inevitable, then let those willing to fight face the consequences, not those trying to live their lives. It might become a worse situation considering the loss of civilian life if invasion took place, but I still see invasion more favorably than to being obliterated.