The Galaxy S6 comes with solid battery life that's on par with the iPhone 6. The iPhone 6s is rumored to be coming with battery life improvements which means that it could hold its own or perhaps beat out Samsung's flagship in testing.
A shift to a new Qualcomm chip could help the iPhone 6s deliver improved battery life. And thanks to some changes under the hood, we could see Apple deploy a larger battery inside the iPhone 6s design. Those, combined with iOS 9's improvements, could also make battery life a huge selling point.
You'll probably have to wait until the device is out to compare iPhone 6s vs. Galaxy S6 battery life but just know that there's potential for something big here.