Beginning of the letter followed by a comma or colon? American or British English?

I don't understand the difference between the beginning of the letter followed by a comma or colon as follows:

Dear Sir or Madam, or To whom it may concern,

Dear Sir or Madam: or To whom it may concern:

My questions are:

1. Is a colon more polite and professional than a comma?

2. Is a comma used in the British letter, whereas is a colon used in the
American letter?

3. Is "Dear Sir or Madam" British English, while is "To whom it may concern" American English?

Your answer would be greatly appreciated.

I can only speak about American English (and Southern English, at that,) but in my opinion,

In American business letters a colon is always used, and used after the following openings, in decreasing order of use:

Dear Sirs:
Dear Sir or Madam:
To whom it may concern:

(In the American South I often open letters to Dear Folks:... it is the language we use and is very friendly-sounding.)

Dear Mr Mercer:

Thank you very much for your quick reply.

I was wondering if you could teach me the meaning of "in decreasing ordr of use".

In the following opennings, do you mean that "Dear sirs:" is more freqeuntly used than "To whom it may concern:"?

Dear Sirs:
Dear Sir or Madam:
To whom it may concern:

I'm looking forward to hearing from you.

Lynn

Here is the rule as stated in various handbooks on American English usage:

In salutations, the comma is used in informal communication.

Dear Joey,
Dear Aunt Ethel,
Dear Mom and Dad,

The colon is used in formal communication, or when you don't know the recipient personally.

Dear Principal Skinner:
Dear Mrs. Flanders:
Dear Director: