Before the ultimate, Nocturne can decide whether he wants to spend it for the dash or the vision denial (in his mind, not in the game). Then he activates the first portion and denies vision:
If he wanted to dash, he will still dash.
If he wanted to deny vision, he won't dash.
Nocturne's ultimate is only more forgiving if he uses it for the vision deny.
Before the ultimate, Twisted Fate can decide whether he wants to spend it for the vision or the teleport (in his mind, not in the game). Then he activates the first portion:
Now TF gains information that affects whether he wants to teleport or not. He might see the person that was 'alone' is actually well-guarded outside of his previous vision or he might find an enemy jungling with low health. He can now decide again whether he wants to teleport or not.
Twisted Fates ultimate is a lot more forgiving if he after activating it for the first time changes his mind on teleporting due to the new information gained. It will greatly buff his ability to look for picks because he can check if there is one and if there isn't, have a reduced cooldown.
This is why the change on Nocturne is strategic and skillful, but on Twisted Fate it would merely be forgiving.