General rules are use your 2h (Nightfall preferably at least until AQ40/Naxx when other 2hs could be better) I kept Nightfall all through in retail.
Why do you even talk about Nightfall preferably until aq40/naxx when this is PVP topic? it's damage is too low in order to kill something even with the proc.
there are much better weapons than nightfall (perhaps ashkandi or legendary mace) (but that's my opinion) . Such as Polearms.