It's I vs It's me

The I in "It's I." is a subject complement. Subject complements are used only with a class of verbs called linking verbs, of which to be is the most common. Unlike object complements, subject complements are not affected by the action of the verb, and they describe or explain the subject. In this case, I is not affected by the action of the verb is, and it specifies exactly who the subject It is. The subject complement therefore takes the subjective case. Usually, this makes no difference in the sentence because English nouns no longer distinguish between subjective and objective case. But English pronouns make the distinction, and the subject complement takes I instead of me. It's I sounds strange to many English speakers, but is considered correct by prescriptivists. In other contexts, the subject complement may sound less strange, such as "This is she" rather than "This is her."

At this point, the use of the subjective in the subject complement has almost entirely disappeared. Both usages are still current, but the use of subjective in the subject complement is much less common.

It should be noted that the use of a nominative complement ("It is I") is by no means universal in other languages. For example, French-speakers say "c'est moi" (it's me) not "c'est je".

Which one is most commonly used for formal and informal? BrE ok