Hiroshima/Miyajima or Koyasan?

I'm planning to travel around the Kansai area for 10 days in June, and was wondering if I should visit Koyasan or Hiroshima/Miyajima instead? I'll be using Osaka as my primary 'base' (because of the flight), and I definitely want to go to Ise/Mie, Himeji, and optionally, Nara. The two itineraries I'm looking at are (number of days in brackets):

Osaka (1 1/2, including arrival) - Himeji (day trip) - Koyasan (2) - Nara (day trip) - Mie/Ise (2) - Osaka (1 then flight out)
Osaka (1 1/2, including arrival) - Mie/Ise (2) - Nara (1) - Hiroshima/Miyajima (2) - Himeji (day trip) - Osaka (1 then flight out)

Which would you recommend (I'm sure I'll go back to Japan again for vacation and will do the one I don't this time then), or does anyone have any other suggestion? Thanks so much!

It stands out that you are not visiting Kyoto at all. Maybe you've been there before?

Personally I'd spend a night at Koya-san (I dont think it needs 2) and then a night at Nara. Nara is really pleasant especially for an early morning walk around the park.

Hiroshima and Miyajima are nice too. The dome in particular was more of a signiificant thing than I was expecting. Miyajima is fun enough, but again it is better to stay overnight, and in a ryokan. The actual tori itself is best viewed at a distance but the approach over the water is nice. The deer at Miyajima are pretty mangey compared to the ones at Nara, though.

I think you just have to choose yourself :)
Everybody prefers different things.

Two nights in Mt Koya is too much though.

You are already visiting Himeji, so it would be quite natural to continue from there to Hiroshima/Miyajima for two nights. Both are quite lovely but I enjoyed the vibe in Hiroshima a lot. There was something very respectful yet peaceful in that city. For me, it was not an option to miss Hiroshima, it is such a big part of world history.

Thanks for the quick responses!

@AK: I was in Kyoto last year and spent a week there (and a week in Tokyo and another in Kanazawa/Kamikochi, and 2 days in Hakone). I loved Kyoto a lot - actually all those places - and even thought about going back again this year, but decided that I should give other cities a try too since I might like them just as much. :)

@Lazy Pious and *: Thanks for the tips! It's good to know that Koyasan only needs one night because I was thinking that 2 nights are 'required' to justify the time going there - then I think I'll really do only one night there and a night in Nara! :)

Koyasan as nice a place as it is.
Only really needs one night.
It gets cold at night due to how high up you are. so would recommend wearing more clothes or a coat.