Powering MRK1000 with Uno, or vice versa

I'm trying to power one Arduino with the other for a my project so that I only need one power source instead of two. I already know how to power each by 5V --> Vin and GND --> GND. Is it better to power the MKR1000 with the Uno, power the Uno with the MKR1000, or is it personal preference?

How is the project powered?