I would use either depending on the nature of the writing. I typically use "Kind Regards" or even more simply "Best" for someone I know.
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You ask about using 'faithfully' and 'sincerely' if you know someone; by this I take it you're asking about using those modes of address in formal or business correspondence (in personal correspondence, anything goes, literally!)
Generally, the exact mode of address depends on the relationship between the sender and the recipient. Sounds obvious doesn't it, but it rea