I’m planning to build an SL-4 for next year and I’ll be going down the DA mechanical route, 11 speed.
2 reasons for this, and the first one is purely aesthetic I’m afraid! The Ultegra Di2 looks shite! They’ve built it down to a price and it shows: massive servo/motor, looks clunky and ugly. I wouldn’t want it anywhere near a £2k+ frameset!
Secondly, the reviews I’ve read of the 11 speed mechanical groupset suggest it makes significant performance improvements over 7900 DA. The shift action is something like 40% lighter – one reviewer said it’s almost as effortless as Di2.
And in a year or 2, I can always upgrade to DA Di2 with new shifters and mechs…