I too like Nagasaki more when comparing the cities themselves, although Hiroshima is right by Miyajima which is a very significant attraction. Hiroshima is just another city asides from the peace park, but it does have a pleasant layout and vibe and its long shopping arcade street is great to walk on as you head from the Peace Park towards Okonomimura. Nagasaki has a pretty stunning hilly landscape dropping down to the sea, and it's a really unique city that blends Western/Chineses/Japnese architecture and history. But the city itself is pretty compact and you can see just about everything within a day. I love Hiroshima-style okonomiyaki, whereas I feel like Nagasaki's famous foods (kasutera, champon) are nothing special. There's a lot more shopping in Hiroshima, although Cocowalk is a good place in Nagasaki... maybe that's not that important. Miyajima is a pretty mesmerizing place to visit and should be combined with a visit to Hiroshima... to me, it's one of the crown jewels of Jpn attractions. Nagasaki does not have a nearby attraction that would rival Miyajima in importance, although there's a lot of pleasant day trips (Shimabara/Unzen, Kujukushima, Haus ten Bos, to name a few).

Hiroshima has the more famous peace park with the iconic Atomic Dome next to it.

Both cities have peace park and trams.