Not so simple to compare...
1) In modern games, the R9-390X is 10% faster at 2560x1440, and only 4% faster at 1920x1080:
https://www.techpowerup.com/reviews/AMD/RX_480/24.html
2) The RX-480 architecture improves on the GCN design so will get a boost in future titles. (It's slightly faster in Hitman and RotT for example so the newer games start to benefit the newer design).
3) The RX-480 is better suited to VR titles (once the software starts getting sorted out).
4) *There are other differences including improvements to:
- HEVC
- overall DP/HDMI bandwidth
- HDR support
The above mainly apply to better monitors.
5) Room temperature.
Not just power draw, but room temperature is an issue to me. My room gets too hot about half of the year so any reduced power draw helps.
So...
Summary:
There are PROS and CONS here. Unless the price is very close, i don't know if the R9-390X is worth it. If more expensive I'd be suggesting a GTX1060 instead.
If VR is of any future interest then get the RX-480 because it will run better. Their lower-level access allows superior asynchronous time warp to help avoid feeling sick in VR (Google that if you want).