Don't think you can go wrong with either one.  I would check with the makers
of the chair lift to see if they know of any issues with their gear in one
vehicle or the other.

When we bought our Odyssey earlier this year the decision came down to
seating.  With leather, the Toyota was limited to 7 passenger.  Had to have
cloth if you wanted 8.  The Honda has 8 passenger with leather.

The last time Consumer Reports rated vans, the Sienna came out on top--  
though by a small margin. Take a look at the reviews and see which features
that are important to your are on each-- then you can do your own
weighting...

get the one with the greatest gross vehicle weight. I suspect it will
be the Sienna because of its solid rear axle.

The chair lift you want to buy is a Harmar AL600 plus 2 optional brackets
they make available. for use with fold down 3rd row seats  You won't have to
drill any holes with that model into your new van.  It fits in the odyssey.
I suspect it fits in the Sienna too.

http://www.harmarmobility.com/products/itemDetail.cfm?proID=31

Also if you are reasonably in good shape you could start off with a ramp.
They are available on ebay cheap.  I bought a folding ramp for about $150 on
ebay and have been using it for 4 years.