I agree with Ruthi. 'Chill out' is an expression which became popular in the hippy era of the 1960s. You can still use it, but these days young people tend to say 'chill' more than 'chill out.'

They're both common, modern street-slang and mean the same thing - relaxing.

Both correct I think the first is more modern and more used.The second was used more in the past but you can say both of them.