I'm biased...as I've just gotten an Ultegra Di2 equipped bike...but if you believe GCN, they say you can't even really tell the difference between Ultegra Di2 and Dura Ace Di2 in the new generation. And based on how crisp and smooth and instant my shifts are, I believe them. It doesn't seem like it could get any better than that. I had a 105 equipped bike before, and even though it seemed nice at the time...the difference between 105 and Ultegra Di2, to me, seems very significant. I'd recommend it 100%. That said, I too was looking at Orbea and Scott and Canyon and Felt...and well, at the end of the day, you can get a very nicely equipped Orca with Ultegra Di2 in your price range. That said, if you buy Scott, you can definitely get a bike with Dura Ace Di2...as I just saw a "new" last year's model for like $5,800 with this groupset at a local shop (Y's Road).
?SCOTT ADDICT RC********************NEW**************************************************??6.45kg**********??? | Y's Road ???
They had it discounted, and then discounted it again. So there are deals to be had on previous year models, if you're set on a specific groupset.