Despite contrary belief, I use them situationally.
Newb: When I'm chatting in a friendly manner to a friend thats struggling.
N00b/Noob: When I'm chatting to someone I don't like who's struggling.
Nub: When I'm having a laugh with people about something stupid... usually during one of the regular Chuck Norris vs Vin Diesel conversations in Barrens chat (god I hate those, I wish the convo would just finish).
And while we're on the subject of Nub. Is it pronounced as newb or like as though it rhymes with club (unless you pronounce club as cloob). I keep hearing the latter whenever I go to a net cafe.