Sunrise Izumo/Seto or JR overnight bus

I'm thinking about taking an overnight transportation from Osaka to Tokyo on Jan 1st with my JR pass. So far, I've found the Limited Express Sunrise Izumo/Seto and some overnight buses as the best options. Has anyone been on one of them? What are they like? Which one is more comfortable? For the train, is it like a lot people sharing a room or something else?

Sunrise Izumo/Seto is definitely more comfortable.

There is a mini-lounge place. You can even buy a Shower Card from the conductor for 300yen or so.
(up to 20 cards for Izumo and Seto respectively)

If you select a Type A or B compartment you don't share rooms with anyone. Other types, you get your own bunk or your own small space.

http://www62.tok2.com/home/tsubame787/seat_285.html (in Japanese but you can see the photos. you'll get the idea)

http://en.wikipedia.org/wiki/Sunrise_Izumo/Sunrise_Seto

If it's a bus with 3 seats per row, it's decent. If you're on a bus with a 4 seats per row format it's a bit rough.

cost aside, it's the Sunrise!

I called JR West to find the Sunrise Seto-Izumo leaving Osaka at 0:34 on 1st and 2nd of January is completely fully booked in every class.

You might consider taking an overnight bus instead.
Even in that case, book a seat as soon as possible.