There is no difference in writing ability between the two forums. Both expect in terms of their required standards accurate grammar and spelling, but nobody is going to eat you alive if you let a typo slip. 

While Advanced is more thought-out and developed than Casual with longer posts, but none of these things are 'writing level'. Using the Advanced Section doesn't make you, or mean you are, a better writer than somebody in Casual: it just determines what you are looking for.