Basically, they both mean the same thing, but may be used in different situations. (A) If you want to say the "the member of police organization": The word "員警" is a formal word, and normally used in formal writings, such as government regulations, formal articles. News reports may like to use this word because is sounds formal. People use "警察" in daily conversation and any writings. (B) If you want to express "the police organization", just use "警察" in most situations. Hope it helps