There’s a lot of choices out there, and I’m wondering what’s the best choice. I’m thinking about SRAM - the little SRAM batteries look like they’re easier to charge, plus you can grab more than one gear at a time, unlike Shimano.
Electronic shift systems
Your statement that Shimano DI2 can only be shifted one gear at a time is wrong. Not only can it be set to shift continuously for as long as you hold the button down, you can set the time interval between each shift (faster or slower).
Also, I beg to differ on Sram’s battery charging being easier. Initially you’ll have two batteries to charge and two to install in the shifters, vs one battery to charge and zero to install for Shimano. After that, you will be recharging at least one Sram battery every 60 hours (approx 1000 miles), while my DI2 battery is good for around 2500 miles. It doesn’t take a rocket scientist to see which is less labor intensive.
Those aside, my biggest complaint about Sram’s system is that you can’t shift front and rear simultaneously. When approaching a sudden elevation change, up or down, I like to dump the rear derailleur two or three cogs while shifting to the appropriate chainring at the same time. That isn’t possible with Sram electronic shifting. I’ve yet to read a review that points this out, but it’s always mentioned in the comments section so I I’m know I’m not the only cyclist who considers this a design flaw
Long story short, the only advantage Sram has over Shimano is that their system is wireless, which doesn’t outweigh the disadvantages, in my opinion.
In all my years of racing i have never once felt the need to shift front and back at the same time, nor have I seen anyone do it, so I think you may be in the minority on this one. You’re right about the charging though - it is more involved than Shimano, but I will say this: how often do you charge your phone and laptop? I have to every day so I think I will be ok with the SRAM 
You are very much correct about serious riders or racers not shifting front and back at the same time; doing so is the best way to drop a chain, I watched it happen on a di2 bike going into a climb. As to the battery thing I much prefer the smaller easily removed batteries of etap to the single point of failure di2. If you have a battery failure on di2 your complete system is down, with etap you can swap the front and rear batteries should one go out, also if you are running the more preferred seat tube battery on di2 this means your bike must be stored near an outlet, and with the size of etap batteries you could carry a spare if you’re paranoid. To me bringing a battery that takes 2 sec to remove and only 45 min to charge is worth it for the added benefit, after all I’m already bringing in and charging my garmin.
It might be that I’m a Shimano guy at heart, but I feel the Di2 is where it’s at. My wife currently runs SRAM, and she dreams for the day that she can switch to Shimano.The technology that bikes are seeing over the last few years is amazing. I still prefer my trusty non electric Ultegra though. It’s just one less thing I need to charge before a ride. I won’t knock SRAM, but for the money, I’ll stick with Shimano.