[lynn]

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.