The Octa-core CPU Rockchip RK3399 includes a gigabit ethernet interface. This alone is a no-brainer win point for the RK3399, as you will only benefit from the speedier network access, particularly against a NAS containing your multimedia library, in glorouis 4K 10bit encoded format. 
The RK3399 chipset includes USB 3.0 type C compatibility, marking a clear winning point against the USB 2.0 interface the Amlogic S912 chipset has.
On the GPU (Graphics Processing Unit) the Rockchip chipset includes a MaLi T864 processor, while the S912 has a slower, less capable MaLi T830 processor.
Amlogic S912 is an advanced application processor designed for performance 4k Set Top Box (STB) and gaming high box OTT/IP applications. It integrates a powerful CPU/GPU subsystem, a secured 4K video CODEC engine and a best-in-class HDR image processing pipeline with all major peripherals to form the ultimate low power multimedia AP.
The Amlogic S912 is a powerful SOC (“System On a Chip“) enabling 4K video in 10 bits -or even 12 bits Dolby Vision- encoded in h265 or V9 compression engines.
If you’ve been following my earlier articles, surely you are acquainted with the Amlogic S912 chipset. It’s specs are probably the holy grail for media player tv boxes at the time of this writing.