Hello! Just adding a little bit here...both of these phrases are correct ONLY if you use the word "seat" rather than the word "sit." EDIT: After reading the other answers, I am wondering if this is a correct phrase in British English? But in the US, this would be considered incorrect with the word "sit" and correct with the word "seat."
In addition, there's another reason not to use the grammatically incorrect phrase "take a sit" -- it sounds very, very close to a vulgar term for the act of defecating, if one replaces the initial "s" with "sh"! So, not only is "take a sit" an incorrect phrase, grammatically speaking, but it sounds too much like "take a s_it!"
Just thought you might want to know that little tidbit of common vulgar speech. :)