If you drive a van 200,000 miles a Sienna will save around 4500 gallons of fuel. (Assume 36 mpg Sienna, 20 mpg Honda) The honda could do better if its lots of road trips (hiway is 28mpg, city is 19). Depending on where you live, and how much gas goes up the next 10 years, I will just "guess" 5.00 per gallon, its likely to be more even this year! That means the Sienna will save $22,500 over the life of the van (200,000 miles).
Given a real possibility that gas could well go "way, way up" from that, the Sienna is almost certainly a better long term buy than the Odessy, counting fuel.
In the short term, the Sienna is also better because the resale value of the Sienna is way better than the Honda, so it will cost you less to own it either way.
There is a reason why Sienna's are so hard to find, and the used ones often go above MSRP.
Even worse: Honda is almost certainly working on a minivan hybrid..they already have a great CRV hybrid, and accord hybrid. Then you would be stuck with a gas guzzling mini van no one would want, and that, too, would kill your resale value as soon as Honda introduces a new hybrid mini van, which may not be that long.
If you add on the additional $22,500 for fuel, the Honda, if you could find one for $40,000 winds up costing you $62,500 for a Honda mini van including the additional fuel cost vs the Sienna. This is probably a best case senario, we already had $5.00 per gallong and it will likely be much much much higher in 10 years, if your van lasts that long. So, the actual cost would likely be $30,000 or more for the life of the van in additional fuel costs on the Honda vs the Sienna.