I pretty much agree with evergrn. Hiroshima has a lot going for it, and okonomiyaki and Miyajima are two of them. The peace park's a nicer place to be too, although the monuments in Nagasaki are cool.
Nagasaki takes a lot longer to get to, but it's definitely worth a visit. Hiroshima is on the way and you could probably do the main sights in a day, but it would be a push.