Geforce 1080 is a no brainer. It's more energy efficient. Unless of course there is an AMD-specific feature you want?
A high-quality 500w PSU could handle Vega just fine though. I must stress the *high quality* aspect.
However, you may not be running your PSU at it's optimal efficiency curve in such scenario's.
I must ask though... What resolution display you running? Might be better to drop down to a Geforce 1070 instead if it's only 1080P.
--::{PC Gaming Master Race}::--