Travel between Osaka and Hiroshima

I am going to Japan for the 3rd time in December and my travel companions are insisting on going to Hiroshima, none of us have been to Hiroshima before. I'm thinking we will probably overnight there however I was hoping for some opinions. Is it worth spending more than one day there if all they want to see is the Peace Memorial? And any suggestions regarding what else we could see...

Also does anyone know how long travel time is and approximate cost of a return Shinkansen trip between Osaka and Horoshima?

Is it worth spending more than one day there if all they want to see is the Peace Memorial?

Hiroshima City alone does not have enough attractions to keep the average tourist busy for more than one day. However, Miyajima Island, just outside Hiroshima, certainly justifies an additional day:
http://www.japan-guide.com/e/e3401.html

Also does anyone know how long travel time is and approximate cost of a return Shinkansen trip between Osaka and Horoshima?

These basics things are written about in the sightseeing guide of this very website:
http://www.japan-guide.com/e/e3404.html?aFROM=2157_2160

I ha just done that within a day
I travel fr Osaka – Okayama – Hiroshima - Miyajima - Osaka all within a day.
Train fr Osaka to Okayama is 1hr by SHINKANSEN & visited castle and the garden ( close to each other )
Then took Shinkansen to Hiroshima & visited the Peace Park & the atomic bomb disaster.
After that took train to MIYAJIMAGUCHI to visit the Shrine in the sea
Must buy JR pass
Victor Wan

I stayed there for four nights a couple of years ago and really liked the place. The trams are a hoot. It would be silly to go there without visitng Miyajima. The floating Torii is an icon of Japan and fantastic. Make sure you visit when the tide is in. There is a cable car up the mountain which has great views if the weather is good. On a more somber note the atom bomb museum is the saddest place I have ever been. I was in floods of tears by the end. However I do not regret visiting it in fact the reverse.

You can add a half day to your visit if you include Iwakuni with its arched bridge.

I was based in Kyoto and it took 2 hours to get down to Hiroshima on the Shinkansen (direct train). Arrived at 10am in Hiroshima, then went straight to Miyajima on the local train, then across to the island on the JR ferry. Miyajima island is absolutely worth a visit, it is incredibly beautiful, and extremely peaceful, one of the best days of our trip. We then returned to the centre of Hiroshima in the same way. Then an overnight stay in the centre of Hiroshima.

The 2nd day we went to the peace park, museum, a-bomb dome etc. It was a gloriously warm day and the park was magnificent. The museum is worth a visit, but really hits a nerve with what happened, but doesn't linger unnecessarily on things. The park is nice to walk round, and the dome itself is moving. However, I think a day for the park etc is more than enough.

You could do Miyajima and the park in a day, but it would be rushed and you wouldn't get to fully appreciate either of them.

I think 2 days is good. You should give yourself plenty of time to appreciate the Peace Museum and Peace Park. It's just not the kind of museum that you want to walk through without stopping to look at things and read descriptions.

Miyajima, as others have said, is also worth visiting. The shrine is beautiful and iconic and you can also walk up the mountain trail.

Other sites in Hiroshima you can visit: Hiroshima Castle, Shukkeien Garden, many museums, and Fudoin Temple in the north of the city. You can get some ideas at wikitravel's Hiroshima page to help you decide what to see or if you want to stay:
http://wikitravel.org/en/Hiroshima#See

Thanks for everyones help. Yes i'm deffinately doing Miyajima, Hiroshima Castle, Shukkeien Garden and ofcourse the Peace Park. And i'm very interested by the bridge that was mentioned, looks really cool. Thinking of staying for 4 days now :)