I have tried both and I now get the display working but as I mentioned USB does not want to talk to PC...
Probably what I should do is to try to power the display separately.
Note: When I tried it using USB, it is off my powered hub, no PC complaints, but maybe internal to HUB shuts off communication.
External power is from wall wart that outputs 5v at 4 amps...
Will try more soon.
I have not had issues powering over USB; however, I am using the 7” screen (on a T4).