たるにTo be (in a position of), As a, For

Both are formal forms of 'to be' used in writing. である is the standard formal equivalent of だ, e.g. 彼は学生である ('He is a student.') たる emphasizes qualification or the qualities appropriate to a role, e.g. 指導者たる者 ('A person worthy of being a leader')

Both relate to identity or role. たる is classical and formal, emphasizing the qualities expected of that role, as in リーダーたる者は責任感が必要だ (A leader must have responsibility). としては emphasizes perspective from a role or position, as in 教師としては当然だ (As a teacher, it’s natural).

たる and として both mean 'as (A)' or 'in the role of (A)', but たる is formal and often used when describing what someone in that position should be or do, e.g. 指導者たる者 ('a person in the role of leader'). として is more neutral and can be found in everyday speech, e.g. 学生として ('as a student').

While these two grammar points appear similar in form, they differ in origin and are used quite differently. たる primarily follows nouns and most often occurs in patterns such as リーダーたる者 'as a leader,' to express qualities or characteristics befitting someone in a certain role. In contrast に足る, can follow nouns or verbs to express the idea of being sufficient or worthy of something, e.g. 信頼に足る友人 'a trustworthy friend' or 語るに足りる発明 'an invention worthy of discussion'.