I have a SRAM eTap groupset on my roadbike and really enjoy it. The batteries are super convenient, I only charge them "defensively" once in a while and have never actually run out on a ride. If I had, I could have swapped the one of the front derailleur, which is significantly less used, with that of the back derailleur, and ride another few hundred (thousand?) kilometers. You could even carry an extra battery just in case. From what I understand, to charge Di2 you need to fully stop and couldn't even charge with a portable battery while moving, but I could be wrong. Seems like SRAM isn't as feature-rich, but is more straightforward and reliable.