I was shopping for a van for a few years and I have decided I will get a Sienna.
Like you, my choices were narrowed down to a Sienna or an Odyssey. I read pages of reviews and asked friends. I've test drove both. I liked both vans.
The Sienna got the vote because my wife bought a Camry in 2002. We really like the dealer. They consistently get good ratings from customers. So, in the end, I figured a Sieena would be easier for us to maintain, and a dealer/shop that I trust. My $0.02.
I use to maintain my own vehicles, but now I'd rather just bring it in.
Another minor yet unforgettable issue regarding the Odyssey was when they first came out with the "Stow-away-seats", Honda dealers wanted around $1K to as much as $5 OVER the MSRP for their vans. They quoted me something like a "real-value-worth" stuff. Left a bitter taste, and I just figure the other vans would copy it. Now all vans have this feature.
I haven't bought a van because I really do not need it-yet. Now, there is speculation that the Sienna will come out with a hybrid in 2007. so I figure I'll wait until 2008 and buy a hybrid. In the meantime, I just go the my Toyata dealer and rent a van whenever we need one.