Originally Posted by JohnJ80

You really don’t need an extra battery but if you do want to carry one, it’s no big deal because they’re pretty tiny. That does allow you to run the battery to empty since it’s the RD that goes dry first. Then you can put the new battery in for the RD and generally finish your ride with some to spare. That option is not available with the Di2 systems especially if the battery is internal to the frame. When it’s dead, it’s dead. On both systems, I’d generally just charge them every two weeks whether the battery was low or not and then not worry about it.

When I talked about connections being a reliability issue in electronic systems, that’s a general statement (I’m an electrical engineer) and “connections” in context is connectors where it isn’t a soldered connection. And that’s true here as well - if you don’t get the connector on right, if it gets damaged, water intrusion etc... there are a number of failure modes that impact connections and Shimano’s cables/connectors while good, are not immune to any of these problems. Especially so if, like me, you travel with your bike and need to dissemble parts of it. I was always worried if I damaged a cable I’d be sunk unless I carried all of the 3 or 4 lengths of cables as spares. It’s nice to not have to worry about that.

Wireless is the way to go and will be where they all are eventually. There is an elegance to it and it’s much less cluttered/aesthetically pleasing on the bike with less cables. All that said, for me, the electronic shifting of both companies has been much more reliable than mech and has needed pretty much no adjustment after it was initially set up. So I’d see either as a step up, but if I had to pick one, it would be eTap over Di2.

For me, riding when temps are often around freezing in spring and fall, there is no question that the eTap shifters are much easier to operate with bulky full finger gloves than are the Di2 shifters.