Hi!
You can use "καλό βράδυ" and "καληνύχτα" interchangeably. They mean exactly the same.
"καληνύχτα" is much more common, though, because it is shorter (one word). Generally, in oral speech shorter forms are used more often.
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Vasiliki Baskou, Instructor/Director, https://learn-greek-online.com.