I just bought and Odyssey and I like it a lot . Has real good pickup(0-60 in about 7.3 sec.) Cavernous interior and handles surprisingly well. I got the low end model and it doesnt have any of the bells and whistles on it. A friend of mine has a new Sienna and likes that a lot as well. I drove both and the best way to describe it to you is this way: Honda gives you a lot more feel for the road, while the Sienna is kinda floaty. No biggie, but that is about all the difference I can tell you. The Honda does have better acceleration than the Sienna, though not by much.
Keep in mind that the Odyssey is supposed to be changing over for '05. You may be able to score one a bit lower than what Honda dealers like to gouge you for. The Sienna, otoh is brand new and I've heard they wont deal too much on price. You really cant go wrong with either vehicle,imo.
Peace
Lounatik