Thanks everyone - I am pretty sold on an 8 passenger at this point, but it's a matter of convincing dh.
Finances are also a factor, and of course you can get a minivan with comparable age/miles for much cheaper if you go for a 7 passenger (since the only 8 passenger vans also happen to be the more expensive ones). So we'll see what we end up with. I very much appreciate all of your input!
We're shopping right now as well, just a bit ahead of you in the process.
While the 8th seat was an option (or standard?) on the higher trim levels of the Odyssey, 8 passengers was only optional on the bottom two trim levels of the Sienna. I'm talking 04-10 Siennas and 05-10 Odysseys, because that's the generation we're looking at.
Odyssey trim levels: LX, EX, EX-L, Touring. 8th seat available on EX and higher trims.
Sienna trim levels: CE, LE, XLE, XLE Limited or Limited in later years. 8th seat available on CE and LE only.
We went and looked at these vans today, mostly looking at 7 vs 8 passenger. We don't really care for the Odyssey 8-pass, because the 8th seat is narrow and uncomfortable. It would be doable if you had the right carseats or for a small to average adult for a short trip, but no more. The third row also seemed narrower, and had hinges placed even further in so RFing seats might not work so well. We much preferred the Sienna's 8 passenger.
DH was very resistant to 8 passenger until we went to look at them. He wanted to be able to easily get into the van to buckle kids without standing outside in the weather, and he thought that wouldn't work well without an aisle. Now he's seen how easy it is to tumble a 2nd row seat, and how readily a 2nd row seat comes out. He's thinking we'll take a 2nd row seat out and store it in the house somewhere, to make a walkway. Then we still have the option to put it back and have 3-across and have the third row seats down for cargo area if we want.