S6 / S6E all the way.. There is no freaking way you will get better batterylife from G4, unless you have a lemon or don't know how to use a phone. Therer is no debate, it's marked on stone that G4 has much worse battery life.
Now before everyone jumps on me and declares me as a troll, read the following properly before you comment.

Source:
LG G4 Review - Accuduur: niet voor de grootverbruiker - Tweakers
For movies, the difference is like double.. LOL
WTF LG was thinking?? Are they freaking serious? They should orice the phone half of what S6 is selling
But I know, most will say that's just one site. Right?

Source:
LG G4 battery life test: nothing's changed since the G3
Ohh, PhoneArena must be paid by Samsung. So obviously they must be out there destroying the image of LG.
Note: GSMArena tests batterylife at 50% brightness.
G4 (at 50% brightness) - 108nits
S6 (at 50% brightness) - 208nits
Meaning S6 is much brighter than G4 at 50% brightness, level at which GSMArena tested the phones. Imagin at equal brightness, the difference will be much higher in favour of S6.
And this is the result with a much faster processor. With a much slower processor, the G4 was supposed to give much better numbers.