Here's my reply to a similar question a while back:
"It depends on how many seats you want. We are about to turn in our 8-passenger Odyssey (it was a 4-year lease) and we just bought an '08 8-passenger Sienna. There are pros and cons to both. If you have a husband who cares about "get-up-and-go-ability" then you need to look at an '07 Sienna or newer and an '05 Ody or newer. When we were looking in '05, the Sienna had a lot less power (which lead to our getting the Ody, along with other reasons). They changed the engine a bit in '07.
It'll also depend on how important leather is to you. Honda's packages make sense to me with what you get each time you get a "higher" package. Toyota's don't make much sense at all. If you want leather you'll have to either get a 7-passenger Sienna or pay the $$$ to have them re-upholster an 8-passenger. The Odyssey comes w/leather no matter what once you hit that "package level."
The Odyssey's 8th seat is pretty useless, so I definitely wouldn't get it if you are going to need that 8th seat.
The other weird thing is that with the Toyota you can only get 1 auto-sliding door if you're getting an 8-passenger. The dual-auto-doors only come on the top-level 7-passenger (SOOOO stupid, if you ask me). I know it seems silly, but I really miss having both doors automatic. The Odyssey also comes with dual-auto-doors at a certain level, but I don't think it ever comes with just 1. I really don't get why Toyota does that.
The other thing is if you're ever going to need to do a 3-across (on either row), the Sienna far surpasses the Odyssey in that area.
There are other minor differences too. The Odyssey comes with more "creature comforts," but not too many things that I can say I miss terribly. I like the "looks" of the Odyssey better (interior and exterior). I don't know if any of this helps at all, but if you have specific questions feel free to pm me."
ETA: I wanted to add that I agree w/pp about the '05 Odysseys. They did a better job with the later models.