Yeah it's confusing.
Some textbooks just say that a negative form makes a sentence more formal.
You will see such an expression in a Japanese exam.
So both of them are correct.
In my opinion, if someone says "koohii wo nomimasuka", the person doesn't know whether you want it, thus you can decide if you take or refuse it. On the other hand, if someone uses the negative question, the person suggests it to you and expects you will take it.