My opinion-
I would also choose the 2004 Sienna, if only for the side curtain airbag system and the vehicle stability control. The great seatbelts and LATCH positions are about the best of any minivan. That assumes that its crash test results are as good as the old version. The 2004 Sienna also has very nice convenience features the Odyssey lacks.
Anything can happen with crash testing, so if you have time to wait for the results on the 2004 Sienna, that would be ideal
Otherwise, the Odyssey is a very good choice also! I would choose the Odyssey over the 2003 Sienna, mostly for the extra mass and length and the better rollover stability.
Really, both the 2003 Odyssey and 2004 Sienna are probably at the top end of the safety spectrum for all vehicles, and the overall differences are pretty minor. The same is true for minivans like the 2003 Sienna, Sedona, Windstar and MPV.
I am very happy with our 2001 Odyssey, but will consider trading on a 2005 Sienna. By then the crash test results will be available and any 1st year bugs should be resolved