I would go with DI2 for sure. I have tried 6800, 5800, 4600, 4500 .... they all shift well. I cannot imagine Dura-Ace would be exponentially different. A little smoother, a little easier, maybe ... but no night-and-day difference.
I'd like to try DI2 just to see ... from what I hear it offers versatility and functionality a little bit beyond mechanical.