During the Manhattan Project the United States developed two different designs of atomic bomb:
I understand why multiple possible designs were pursued. This had never been done before, and they were uncertain what would work out. But they were clearly confident enough in both designs to use them in war. Why was one of each design deployed against Japan, instead of building and dropping two of whichever design was thought to be superior? Is there a reason the plutonium design was dropped first, and the uranium design three days later?
Background links:
https://www.atomicarchive.com/history/atomic-bombing/hiroshima/page-2.html
You've got it backwards — the uranium design (Little Boy) was dropped first (on Hiroshima). The plutonium design (Fat Man) was dropped second (on Nagasaki).
As for why the order — part of it was availability of components. The uranium bomb was just ready to use several days before, and in fact the gap between the two bombs was theoretically larger than it even appeared to be, because weather issues delayed the Hiroshima strike by 3 days, and weather issues also caused them to pull an all-nighter to have the second bomb ready head of schedule.
But it's also the case that they had more inherent faith that the yield of the Little Boy bomb would be impressive. They thought the chance of it failing or having a less-than-impressive outcome was pretty low, because it was such a simple design (and a lot easier to calculate). Whereas the implosion bomb was more complicated and had more possibilities of error, either for no detonation, or for a sub-optimal yield.
But ultimately it had to be either one way or the other, and one could come up with rationales for either. As it was, it was mostly an issue of supplies — they expected to have the uranium ready to go before they expected to have the plutonium.
The implosion design was clearly a more efficient use of fuel (it used 10X less fuel than the gun-type bomb, for the same explosive output), and if they had taken apart the Little Boy bomb they could have made many more hybrid plutonium-uranium implosion bombs. This was in fact suggested by Oppenheimer after the Trinity test validated the implosion design. But it was vetoed by Groves, who cited timing as the main issue — to do it would set their schedule back by over a week and Groves thought that was most important.