A few downsides
Before going into just why the apprentice has overcome its master, I should note a few flaws. First, the manufacturing tolerances used in the production of Ultegra Di2 are not as good as Dura-Ace Di2, which is still built in Japan. This, along with the lack of ti and carbon bits in Ultegra Di2, is a big reason for the difference in cost. It also means that Dura-Ace Di2 will age better than Ultegra Di2, keeping that new-bike feeling far longer into its life. So if you ride your bikes for 10 years, and expect to keep the same group that whole time, Dura-Ace Di2 may be the way to go.
Second, Ultegra Di2 is a bit heavier, but not by much, as noted above. In fact, if one takes out the non-electronic components, it only loses about 104g to Dura-Ace Di2. So run a Dura-Ace crank, brakes and cassette and you gain less than a quarter of a pound while saving thousands of dollars.
Why Ultegra Di2 is better than Dura-Ace
Cost is a huge consideration here. The difference in quality simply doesn’t mirror the price difference between the two groups. And as noted above, with some easy component swaps there doesn’t need to be much of a weight penalty, either.
There are places where Ultegra Di2 is simply better, and more advanced. For example, the number of micro-adjustment stops used to tune the rear derailleur has been increased from 24 to 30 total, meaning more precise tuning.
The aforementioned new cable harness is a nice improvement as well, allowing for smaller holes in frames and simpler setup, plus it’s completely water proof right out of the box. No more shrink-wrap tubing. Ultegra Di2 will also be compatible with a new Shimano diagnostic tool, which will be mostly owned by shops I imagine, and can easily diagnose the entire setup while it’s still on your bike just by plugging it into a laptop. It can pick out shorts or other problems immediately, and send your mechanic looking in the right places for a problem. It’s also possible to change the function of each button, if one desired. Dura-Ace Di2 can only plug in one component at a time.
Last, but certainly not least, is the fact that shift performance on the bike is largely indistinguishable from Dura-Ace Di2. Rear shifting is still crisp and on-command, and the front shifting is astounding coming off a mechanical system. A few journalists who had never ridden Di2 before were giggling with glee as they mashed out of the saddle up alpine climbs while shifting easily between their 39 and 53 tooth front chainrings. Until you’ve experienced the front shift quality that Shimano has brought to both Di2 systems, your life as a tech-nerd cyclist remains unfulfilled.
Shimano does still need to update its accessory shift buttons and TT shifters to the new wiring harness. Once those bits become available, Dura-Ace Di2 will truly be left behind.
It’s up to riders to decide what group is right for them. Red, Super Record, Dura-Ace mechanical, or either Di2 system are all spectacularly good. Ultegra mechanical is great, as is SRAM Force. SRAM Rival is damn near unbeatable for the price.
But if you decide to go electronic, why buy Dura-Ace Di2? I have no idea, really. I wouldn’t. Buy the Ultegra Di2 electronic bits, and pair them with whatever cranks and brakes you want. At least until Dura-Ace Di2 gets an update, it’s simply not worth the extra cash. I imagine the good people at Shimano already know that, and we’ll see something new relatively soon.