I actually did both . As noted above, and unlike the old Pixhawk 1, applying power to the servo rail does NOT provide backup power into the FMU on the Cube. Likewise, the cube does not provide power out to the servo rail. Applying power to the servo rail only supplies last ditch backup power to the IOMCU, which is of little practical use to begin with but I still do it.
- Power brick included in the box with the cube goes to Power 1
- A 5 volt 10 amp BEC from the power distribution board sends power to the servo rail on an unused slot as backup power for the IOMCU, and also to Power 2 as backup power for the Cube, and also to power the HereLink air unit.
On a loosely related note, I found the other day that USB power alone to the cube will not be sufficient to run the cube, Here2, HereFlow, and an SF11/C Rangefinder. All those devices take power from the cube. So if you have all those devices hooked up and are only powering the cube on USB (like sitting on a desk), sensors start acting strange. I was very confused by sensor readings that made no sense. Once I hooked up main power, everything started working properly.