Hello,Daniel. It's a formal way to put it as to say the word"卫生间",while the word "洗手间" is a casual word ,you can use it in conversations with family,friends,colleagues,etc.

Hi,I am royu,a Chinese native.We use both of them to mean toilet in daliy life.Just likes “there is a guy”and“there is a man”in English.

lol，只是叫法不同，其实一样