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According to Phoenix Studio forum (translation) and other sources:
both are based on Android 5.1 Lollipop,
both support Legacy BIOS and UEFI boot methods, but sometimes Phoenix OS Legacy mode does not boot, unlike UEFI,
Remix OS lacks adjustable windows and some of them can only be maximized or minimized,
Add to the above answer; from what I found while testing both OSes:-
Phoenix OS is now x86 only platform, while Remix is x86/Arm
Phoenix OS supports window transparency.
File manager in Phoenix OS makes more sense in use.
Settings panel of Phoenix OS is more well-organised.
Phoenix OS still lacks of Ethernet support. (major drawback).
Phoenix OS doesn't ...
I can add to the above:
In PhoenixOS clicking "X" in the top window bar doesn't kill the app; thanks to that, apps can run in background without icon being visible in the bottom taskbar (like in Remix)
PhoenixOS works with floating windows just fine, no matter of app window size. RemixOS fails at this quite much - works only for some apps and only in ...