Adding to the already-existing multiple variables which @Napoleon PhoneApart mentions come into play when comparing battery performance between iPhone 6s and iPhone 6, there may be variations between individual 6s (or 6s Plus) units that will come to light in the coming weeks as a result of this: Macrumors points out Chipworks' recent revelation that some iPhone 6s units have a CPUs from Samsung, while others have CPUs from TSMC. These CPUs are built on different nodes: Samsung's on a 14-nanometer node, while TSMC's are built on a 16-nonemeter node! What if Joe Schmoe realizes he has an iPhone with a Sammy CPU, and Jane Schmäne realizes she has an iPhone with a TSMC CPU? And what if benchmarks show that Sammy-CPU iPhones consistently perform just a bit higher than TSMC-CPU iPhones? And what Sammy-CPU iPhones' batteries consistently perform better than TSMC-CPU iPhones? This has the potential to become a headache for Apple—and one which they well deserve, in my opinion.