First of all, "Yours faithfully" and "Yours truly" are complimentary closings, not salutaions. The salutation is the "Dear ------" portion of the letter.
"Yours faithfully" is a closing that should ideally be used with a client with whom you have an ongoing relationship, one with whom you've done business with satisfactorily for a period of time. "Yours truly" is a standard closing that is always appropriate.
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