a mild or indirect word or expression substituted for one considered to be harsh or blunt when referring to something unpleasant or embarrassing : “downsizing” as a euphemism for cuts in the number of employers in a factory.
In these sentences, you are referring to whether one expression is casual speech, and one expression a more formal way of saying it.
Neither of the sentences you give is correct.
In relaxed, casual conversation, where I have perhaps just met the person and had a conversation, and now saying goodbye, probably not ever going to see them again (the kind of conversation you may have whilst waiting in an airport lounge with someone sitting near you.): "It was nice talking to you."
With a friend (or acquaintance) that I don't see very often, and I bump into them and we talk for a little while, I might say, "It was good talking to you." meaning that I had enjoyed meeting and chatting with them again.
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