Erik:
I guess I need to beg that you consider the following two things.

1) No where was anybody talking about switching in between the two supplies in a
hot fashion. But...

2) If hot switching is desired any solution will require a bit of care in the design. And NO it does not take a capacitor the size of a beer keg to hold up the power on a typical embedded system. The couple millisecond switching time of a relay is trivially easy to hold up with a reasonably sized capacitor even if the processor and associated circuitry draws the current limit of what is allowed for a USB device.

Let us not try to mislead the people that we give suggestions to.
Mike Karas