Himeji to Miyajimaguchi

With your first route, you are changing bullet trains in Okayama to the sakura bullet train, which has fewer stops, so you will arrive in Hiroshima sooner. It is preferable to the second route since it gives you a half hour more of free time to do sightseeing. Please be aware that there are 2 ferry companies to the island, the JR ferry and the Miyajima Matsudai Kisen. If you are using a JR Pass be sure to go to the JR pier. For the JR ferry the last ferry leaves Miyajima at 10:14 PM. But in actuality the place will become like a ghost town after sunset when all the tourists leave.
The last ferry going to Miyajima leaves at 10:42 PM - but it's unclear why you ask this because the only people taking this ferry either live there or are staying overnight in a hotel there.

Thank you very much Toraneko.

The second route didn't mention it stop at Okayama, and that is what confuse me, thanks.

We are over night at Miyajima, sorry I didn't mention that.

Where can I find more information about those two bullet train? I would like to know which one is more comfortable?

Thank you again.

I don't think one is necessarily more comfortable than the other. You should be able to relax quite well in either. Given that the ride is not long, and your sightseeing time is limited, the faster way by the Sakura is a better choice.