What are the internal changes?
The new iPhones ship with Apple’s new A9 processor, a 64-bit chip embedded with the M9 motion coprocessor. The M9 makes it possible for your iPhone to track your fitness with an accelerometer, gyroscope, barometer, and compass, and also supports the ability for Siri to remain always on. Just say, “Hey Siri” to summon her without needing to press the home button or plug your phone into a power source. The M9 will also add running and walking pace-tracking to the iPhone 6.