Ok, I didn't know there was a difference, so I looked it up. But the definitions I read gave me the opposite viewpoint. Seems to me nub is just somebody who sucks at something and noob is somebody who's new at something.
Let me clarify.
NUB - New Unqualified Body. (You haven't even earned that right, yet, to be called a person (("ipso facto" you have NO personal rights as a NUB)). You are an air breathing, food consuming, waste producing leech until you are qualified. Once qualified, at something, you are now at least human. When you are fully qualified you can now be re-labeled a "person" and thusly are a member of the ship's 'useful and qualified' personnel. And, you guessed it, the only ones that count for any good are those that are not NUB's.)
Noob- Is a newbie or a derogatory term to someone who is not new. YMWV on this one.
This is my personal take from my own experience's.
Now back to my nap (which you can only take if you are fully qualified).

Hope that helps.