The short answer is: neither, and both. (Hey, you were at /r/AskHistorians, what do you want, something straightforward?)

Longer answer: both Alperovitz (a "revisionist") and Maddox (an "orthodox") have marshaled historical narratives based on (necessarily) limited evidence, copious interpretation, and towards very specific, but diametrically opposed, political ends. In looking at their evidence, they bring different assumptions to how it should be interpreted, and they weigh some evidence higher than others. (This is how historians work, so this is not an aspersion on either.)

Let us take a very simple example. While at Potsdam, Truman wrote about the entry of the Soviet Union into the war against Japan in his diary, on July 17th, 1945: ""He'll [Stalin] be in the Jap War on August 15th. Fini Japs when that comes about."

Alperovitz thinks this little line supports his thesis that the atomic bomb was not needed, that Truman and his advisors thought the war would end once the Soviets entered the war. Maddox points out this arrived prior to Truman learning about the atomic bomb test's success. The next day, after learning about the bomb test, he writes in the same diary: ""P.M. [Churchill] & I ate alone. Discussed Manhattan [atomic bomb] (it is a success). Decided to tell Stalin about it. Stalin had told P.M. of telegram from Jap Emperor asking for peace. Stalin also read his answer to me. It was satisfactory. Believe Japs will fold up before Russia comes in. I am sure they will when Manhattan appears over their homeland. I shall inform Stalin about it at an opportune time."

Now, which of these fragments of thought — Tweet-sized shards of a man's consciousness that have been preserved over the ages, through the mediation of him writing them down (and who was he writing them for, one must always ask of a diarist) — gives us the most insight into Truman's thinking about the end of the war?

Alperovitz will tell you fragment 1, Maddox fragment 2. Or something like that. And one can debate it: fragment 2 is later in time, thus it is more representative of how his thoughts changed; fragment 1 is more definitive, because it establishes that Truman thought that Soviet entry would end the war either way. And so on. This is what historians do, they argue over things like this. (It's good work if you can get it.)

OK, you say, I get that. Obviously these are interpretive issues. But who is right?

And this is where we come back to the short answer. There are aspects of Alperovitz's thesis that are compelling, especially when compared to the "old" orthodox/traditionalist thesis, that the Japanese were determined never to surrender, that the use of the bomb was very carefully weighed by those at the top, that it was ruefully agreed that two and exactly two bombs would do the trick, and that the bombs did do the trick. Alperovitz emphasizes that while there was a lot of discussion at the top, it all assumed the bombs would be used once they were available, that the USSR played a large role in some of their strategic thinking, and that we should see this act not simply as a US/Japanese interaction, but a US/USSR/Japanese interaction.

Maddox, however, correctly pushes back on some of Alperovitz's interpretations. It's not as cut and dry as Alperovitz would have it, with mean policy officials scheming about how they are going to shock Russia with the bomb, and completely avoiding all means of Japanese peace "feelers." He points out that the "feelers" to the USSR were very informal, that the terms were never elaborated, and that the evidence that Japan might have been convinced of a "diplomatic" solution prior to the bombing (or Soviet invasion) is very slim. He better emphasizes that there were many on the US side who really did think the atomic bombs would bring the end of the war. And so on.

Who is right? On many key things, they are both right, and it is a question of interpretation/weight. Both are at times grasping in the dark, because we only have a fraction of this stuff preserved, and we don't trust it all equally. They are also both wrong on some points — both assume very strong precognition on behalf of their historical actors (they "knew" Japan could or couldn't be convinced in one way or another, which is all nonsense, nobody "knew" much of anything about the future — people believed or thought things but there are always large uncertainty bars on the future).

J. Samuel Walker, in his review of atomic bomb literature (which is the best "review that compared both as objectively as possible" that is out there), concludes that aside from these two polarized kinds of accounts, most historians were settling on a "consensus" view that splits the difference, taking the best parts of each and merging them into one coherent narrative. I have written about this here, citing his review here. This "consensus" view (we can merge in other interpretations, like Hasegawa's, that argue the Soviet invasion is much more important from the Japanese side than either Alperovitz or Maddox give credit) more or less treats the narrative as messy, full of multiple, multiply-motivated historical actors, working in levels of uncertainty, with many possibilities open. Which is to say, what it does better than either the "revisionist" or "orthodox" view is establish that there is no "easy" or "simple" narrative that either makes the US (or Japan) look great or terrible. Real life, and real history, is more messy than either of these possibilities — shades of gray.

So there is value in both accounts. But take each with a grain of salt. My main methodological issue with bomb scholarship, perhaps because it is so often written by diplomatic historians (who for whatever reason feel the need to self-present as all-knowing), often hides big interpretive jumps within simple statements of fact. When I write on these things, I try to make it clear where the big jumps are — where I am definitely imposing a subjective, humanistic understanding of a fragment of evidence. We cannot truly "know" what Truman was thinking, for example — it is just outside of possibility. (Do we even truly "know" what we, ourselves are thinking? What our spouses and friends are thinking? There is always a big gap there. And for dead figures, with whom we are communicating via scraps of fragments of things that got written down, the gap is massive, and we should not be fooled into thinking that these things are straightforward.) That does not mean we should not try to come up with plausible narratives that explain their actions, and to find evidence for them where we can. But we should be straightforward about the nature of our work, the places where we are arguing that a scrap of evidence is compelling and why, and acknowledge the ways in which we are piecing together an elaborate story based on a very slim number of data points.