eTap apparently still has its issues for too many users. Between that and the need to maintain four batteries, I've never felt tempted to go that way.
As for Di2, I have ridden every variety of Di2 from the beginning -- including the original DA version before etube. Right now, I have Dura Ace Di2 on one bike and Ultegra Di2 on another. There is absolutely no functional difference between the current Dura Ace Di2 and the current Ultegra Di2. Except for coloration and perhaps a few grams weight difference, they are the same. Di2 is extremely reliable and low maintenance -- less maintenance than a cable actuated system. (And there's just one battery to monitor.) I would recommend Ultegra Di2. You can alter shift patterns, button functions, shift speed, etc. to get exactly the shifting you want.