[Maksim Vus]

So if I select "line in" I'm good to go for REW? What the difference between that and "microphone"?

difference only in "Low" or "Hi" sensitivity (Hi - for microphone). But it just "program" parameter.
next step - check level in RAW. If you chose "Line" but level in RAW is too Low, then you can try "microphone". And vice versa.