Of the two, Celsius is less arbitrary because it is based on actual measurable reproducible things and not “we threw in some salts in water, and guesstimated a human body temperature”. It also makes a lot of sense in our post-industrial society because we do/don’t want to freeze/boil water almost every day for a variety of uses. Water is both an extremely important substance for humans and its freezing/boiling points occur in everyday life (unlike air or metals).