in a semi blind comparison, I prefer demucs for all 4 tracks (drum, bass, vocals, and other). bass and other stand out the most so let me say a couple words about them.

bass - the demucs bass has less bleed from other instruments and the volume is consistent throughout. with spleeter, the volume varies a lot and there are multiple sections of 1-2 bars where it just drops out completely. In Capo, the demucs spectrogram is nice and clear whereas spleeter tends to look like pencil smudges for the most part.

other - with spleeter, whenever there are vocals, the other instruments turn to mush. demucs is much better. Oh, you can tell people are singing -- the instruments get muffled -- but you can still hear them.