In 10.12.6, yes, both GPU are OOTB.
The main benefit of the 280X is very good driver optimisation, and able to flash.
If you have not plan to flash it (to show boot screen), then I personally will prefer to RX580 more.
It has more VRAM, it's a much newer card, it has better port (e.g. HDMI 2.0), it has more functions (e.g. hardware h265 decode in Windows), it draw less power, it has higher performance (assume same level of driver and software optimisation).....
Even though the 280X may do better in some particular applications, but the RX580 should not fall behind by too much. And the RX580 usually can do better (by it's raw power) even thought the software is not optimised for it yet.
For production, I think you better go for a new RX580 that come with warranty. But not a used 280X that may die at anytime.