It kind of depends on the motion of the twitch as well. The annoyed one is usually a quick twitch. Usually when they are seated they will keep their tail mostly stationary but flick the tip very fast back and forth.
My cats will flick the tip of their tail around when happy too but is a slower and calmer motion when they are happy.
From what I see this is pretty true from my experiences with cats. It also depends on the cat itself. EG will swish his tail back and forth in a large motion when he is happy as well. Which looks like he's angry to someone who doesn't know him.
