This is a false dichotomy. In the words of the post-war US strategic bombing survey:
"Based on a detailed investigation of all the facts, and supported by the testimony of the surviving Japanese leaders involved, it is the Survey's opinion that certainly prior to 31 December 1945, and in all probability prior to 1 November 1945, Japan would have surrendered even if the atomic bombs had not been dropped, even if Russia had not entered the war, and even if no invasion had been planned or contemplated."
While this is all speculation, that was at the very least a defensible point of view held by a bunch of Americans shortly after the war.
Regarding firebombing: Hiroshima alone killed probably more civilians than the entire Tokyo firebombing campaign. A firestorm is a terrible thing, but you can still run from a fire even if your whole city burns down; you can't run from a nuke.
So if you measure collateral damage primarily in civilian deaths, firebombing still looks much better (a hypothetical firebombing campaign would have probably killed <40k civilians in Hiroshima instead of 100k, guesstimating from Tokyo numbers).
Edit: I don't think dropping the nuclear bombs was especially ethically questionable compared to the rest of the war, but I feel it is very important to not whitewash that event as valiant effort to save young American conscripts. Regarding it as a slightly selfish weapon demonstration feels much more accurate to me.
