A noob question about revoking titles...

I just bought this game a couple days ago and I am pretty much addicted... I've started out as the King of Scotland (completely taken over and created the Kingdom of Ireland and am a county away from being King of Wales) and I've put down my fair share of rebelling vassals (mostly when my king dies and his successor takes over).

Sometimes, the vassal I imprison will have a Duchy and several counties under his authority. Is it better for me to revoke the Duchy from him or one of the counties? I always keep them in prison afterward and let them rot and that's worked out because their regents never rebel or go against me.