Is it me or is it I? easier to figure out than It's me or It's I.

This becomes a bit of an issue of personal style; both in speaking and writing.  If you are already using contractions, you're the casual sort.  Obviously, I am, too.

You're trying to make the right impression as you knock on the door of your blind date's apartment, or on the door of the personnel manager who has your job application. "Who's there?" you're asked. What's your reply? Usually, I say, "It's Susan."  It's rare that I would answer with, "It's me."  That's a bit presumptuous, but you get the point.

If you've ever been unsure of whether the answer is, "It's me" or "It's I," read on.

Pronouns as subject complements.
A subject complement is a word or phrase that follows a linking verb such as is or seems; it's the that in This is that, and it's the gray in All cats seem gray. A subject complement isn't the object of a verb but something linked to the subject by the verb. The rule for subject complements is very simple: They should be in the same case as the subject they are linked to, which is, of course, the subjective case.

It's me and It's us break the rule, but this causes discomfort to many because It's we seem impossibly unnatural to them. Grammar experts often advise breaking the rule whenever the subjective pronouns I and we seem stiff or "affected," as they do following the informal contraction It's and in many other situations.

That was we waterskiing by your cabin yesterday;

When you hear three knocks, it will be I;

His chief victim was I

-- such sentences may obey the rule, but they are quite uncomfortable when attempting to speak them naturally.

and

There are, of course, sentences in which obeying the rule is not an uncomfortable and therefore, easier to swallow.

In It was I who broke your ski, the subjective who seems to make I preferable even though in principle there need be no agreement in case between a pronoun and its antecedent. The ear has to be the judge.

It's him and It's her cannot be defended quite as energetically, because the rule-observing It's he and It's she, though perhaps slightly stilted, are not outlandish; most careful speakers and writers do use them. It's them is perhaps more often defensible, because It's they is more than slightly stilted. Again, the ear must be the judge;

That was he skiing by your house seems fine to most, but His chief victim was she seems contrary to idiom, and to a lesser extent so does That was they skiing by your house.

Our suggestion: When asked, "Who's there?" and you want to sound natural and idiomatic, go with "It's me." But if you are eager to impress with your impeccable grammar, consider replying with "It is I."

Source: The Handbook of Good English, by Edward D. Johnson.and ProofreadNOW.com

Sidenote:  I go with the most comfortable on my ears. I don't want to come off like some spoiled Bryn Mawr brat from the 30's.