I vote neither!
Keep the Zullo. It looks to be in great shape and I think the reputation and following that Zullo has trumps both the Concorde and Duell. Looking at the pics of your Zullo, the riding position appears to be right on the money. There seems to be the right amount of seatpost showing and the bars seem pretty low, giving you a pretty aggressive position.
What would you do with the Zullo anyway should you get a new frame?
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It never gets easier, you just go faster. ~ Greg LeMond