Samsung's Galaxy S6 Edge delivers high quality battery life thanks to an efficient Exynos 7 processor and a large battery housed in the rear of the phone. It does not, however, come with a replacable battery which means you're stuck with the stock version.
The iPhone 6s probably won't feature a removable battery either and it's also expected to bring excellent battery life to the table thanks to its processor and perhaps, a larger battery.
Apple could be shifting to a new Qualcomm chip and if that happens, it could help the iPhone 6s deliver improved battery life. In addition, we might also see Apple deploy a larger battery inside the iPhone 6s design. Those, combined with battery life improvements in iOS 9, could also make battery life a huge selling point.