From my own experience, a few months ago, on Linux mpv was still the better choice (more performant, better image quality, some more polished/reliable features like subtitles) as long as you don't mind the minimal UI (or use a front-end).
I tested this with an mpv developer. This depends on a lot of factors, as you might be used to with mpv, but mpv was overall just as power efficient, if not better. This was done months ago with a nightly build of mpv and a nightly build of vlc, I'd be happy to do more extensive testing later if there is interest, but I decided on a particular power friendly configuration for mpv for laptop usage in the end.
From my own experience, a few months ago, on Linux mpv was still the better choice (more performant, better image quality, some more polished/reliable features like subtitles) as long as you don't mind the minimal UI (or use a front-end).