I had a 8 passenger 2008 Sienna, now I have a 2012 8 passenger ody. I liked the seating in the Sienna way better in most ways - the one thing it lacked was tethers in all spots. And I don't know why they didn't fix that with the redesign - instead they made it worse!
You do not exactly lose armrests for the captain's chairs in the Sienna - you can fold the middle seat down and then you have center arm rests. It is not the same but it works. The outboard seats are so easy to flip to get to the back that moving them is not a huge deal. Of course if you don't need the space 2 captain's chairs side by side are nicer in some ways.
While the 8th seat in the old Ody sort of sucks, it is still a seat, so the old ody gives you the best of both worlds - true captains chairs you can put side by side if you wanted, and an 8th seat if you ever want it.
But ... the old ody did have some weird thing with the tethers in the back row. You had to stretch them across the trunk. New Ody fixed this.
I don't know what the 3rd row was like of the old ody as far as comfort. I do know the new Ody has a ton of legroom and is very comfy for an adult in the 3rd row. It is 3 1/2 inches narrower than the old Sienna but unless you are putting 3 across back there all the time it's not a huge deal. 3 people will fit without car seats, one 1 in and 2 not in. (in the new - old is similar width wise)
Have gone back and forth on going back to old Sienna, I missed my old van a LOT and still miss some features of it. But the new ody has some features I like one of which is the back up camera. While I am still not used to using it all the time it kept me from running over my stroller last week and that in and of itself has made the van worth it to me!!! The back up camera is a nice thing to have
eta all that said if I had to pick between the two I would probably lean Sienna