In the full context I'm kind of surprised there was any kind of split twixt the two given the full context that both H & N were on a very long target list being systematically worked through and both were destined to be destroyed and effectively levelled regardless of whether untrialled prototype nuclear weapons were tested on those cities or not.
As were 72 other cities (including Tokyo) prior to either H or N being touched.
ie. In the full ethical context the deeper question is really about programs of total war / total destruction rather than the edge case of using two targets as test sites for novel weapons.