Thanks, everyone. I went to both in the end.
The Hiroshima Museum is currently undergoing renovations. Only a small part of the usual exhibit is in the adjacent Annex. There were definitely some powerful exhibits there - particularly the survivor testimonies, the before and after overlays of the city and the everyday articles that survived the blast. The large outside area was also very interesting to visit.
The Nagasaki Museum had different things to offer. A little more intimate. Also more context about Japan's expansion and more militaristic tendencies prior to the bombings. I think a little more global perspective is important when trying to make some sense of the incomprehensible. They also have a great sculpture park outside.
