"You must eat more."
Translation:Vous devez manger davantage.
No difference in this context. You may want to use "davantage" instead of "plus" if there is any risk of confusion with "ne... plus" (although pronunciation of the later is plu, while the former is pluss), particularly in speech, because we very often skip "ne".
ex: "j'en ai plus" : only the final SS sound should mean that your mean "more" and not "any more" (pronounced PLU)