Slog3 is drastically better than slog2: it allows for two stops better DR. But slog3 on the a7S II is not properly implemented, and ends up giving no advantage at all (same DR as slog2), it's just a waste of codec color space (that some may want to go for if they're going to cut the footage with, say, FS7 footage).
I meant to say slog3 on the a7sii, in comparison to slog2 from a7s or a7sii.