Welcome and congratulations on your upcoming addition.
You present quite the complex situation. Between the Sienna and Mazda5, you'll get a much smaller van with the Mazda5. At the same time, you'll most likely get a lower price and better fuel economy in the 5 due to a smaller engine and seeing how the 5 is slotted below the Mazda MPV, a competitor of the Sienna.
I don't believe the 5 arrives here until late summer/early fall.
If I were you, I'd treat the Mazda5 as a Sienna replacement rather than a second vehicle unless you really think you'll need the minivan room for both vehicles.
As far as the Scion xB, it's a great value and a neat little car, but it doesn't offer side airbags.
I'd look into the crash test ratings for all of the vehicles you're considering through the Insurance Institute for Highway Safety or the National Highway Traffic Safety Administration.
I hope that was somewhat helpful!