oh sorry to hear that its fully booked, i don't know those other 2 hotels you mentioned, but samurai-san seems to know it..
anyways, I think its better to stay in Hiroshima rather in Miyajima . (1) closer to the stations and city. (2) from Hiroshima you can always take the train to Miyajima anytime..
i found this information :
http://www.jnto.go.jp/eng/ttp/ptg/PS/pg-603.pdf
i know for a fact that some of my friends did stay in Miyajima Ryokan ..