The main difference between them is that Ferdinand is an honorable man who fell in love and Caliban wanted her only for himself and wanted to "defile her honor" and "fill an island with their offspirng", also Caliban wanted to kill Miranda’s father whereas Ferdinand wanted to make him his father-in-law.
His pledge of love is made on the fact that her honor won’t be defiled by such a primal instinct but will be protected and cherished by him until the day he dies.