I just traded in a 2008 8 passenger Sienna with 39k on it. I am gentle on cars... and it was a great car and in great shape but it had a leak in the cargo area that no one could figure out. I HATE the current Sienna compared to the old. We got a 2012 Odyssey and there are so many things about it that are driving me bonkers compared to my Sienna. I want my old Sienna back.
Seating wise though both work well for 4 kids.
I put 2 in the middle and 2 in the back - I have also done 3 across and one in the back, to get in and out the hatch.
I loved how on the Sienna you could tumble an outboard seat forward and strap it in place - you can't lock he outboard middle seats of the odyssey out of the way but ready for use. Those seats are driving me bonkers. (If the Sienna seat broke lose from its moreings so to speak it would just crash down and lock on the floor - but if the girls don't reset the captains' chair right getting in to the back seat of the Ody it just slides back and forth noisily while I drive... drives me bananas that it can't be stored somewhat out of the way but on hand like in the Sienna)
Also very irked that the back windows on the ody don't open - didn't check this when looking because never thought to! One of my girls likes to yell HI at people so she is bummed.
If you can find an older 8 passenger Sienna in good shape - get one. I know I didn't put many miles on that van but I never had one single mechanical problem. I'd trust one with decent miles.
Otherwise I would do new Odyssey.
I am hoping Sienna goes back to their older type layout in a few years and I would go back to Sienna but I just don't like the new ones. That 8th seat is a joke and they don't have top tethers in all spots... and I hate the tracks in the floors.
I am trying to learn to like my Ody but I feel like the interior of the old Sienna was just more intelligently designed. But I do love the top tethers in all 6 spots of the ody. VERY versatile, eve more so than my old Sienna which only had 5 and lacked one in a spot I wanted one. No worry about seat belt installs, I have actually come to prefer the seat belt install - but the 5 most usable spots in the ody have lower anchors, too.