Usurp Duchy or claim in war?

[HERC0]

I can afford to do both, but I'm wondering if I claim it in a war will I gain the last County in the Duchy to consolidate it completely under my rule? Cos I know that if I usurp the Duchy then I will only gain the Ducal title, but the previous Duke will remain as the Count of the last County.

 

[Yenzen]

You will add all the counties in that duchy to your realm which the one being declared upon holds, provided that the claimant is you or a dynasty member or a landed vassal.

 

[Mutineer]

yes, declaring war is better if you have claim. You can usurp title with out claims, whcih is advantage of usurping.