Where I live in Europe, an er9 upgrade kit is still only two thirds of the price of the cheapest offerings for 105 Di2. For that money I think it’s a good offering (if you’re ready for some tinkering, that is - as with most cheap D2C stuff from China). But if that gap narrows considerably, I think I’d definitely go with Shimano.
As for battery life: I got around 750k out of the first charge. Some of it was in ERG mode on the trainer, though. So no shifting.