Somewhat akin to Intel, Apple has trained consumers on its "tick-tock" cadence of releasing a newly designed iPhone and following it up a year later with an optimized "s" variant. That's been the formula since the iPhone 3G and iPhone 3GS were released in 2008 and 2009, respectively.
Blazing fast A10 Fusion SoC; higher-capacity storage options; IP67 dust and liquid protection; improved camera system; Taptic Engine is very convincing at simulating a physical button press; slightly larger battery
Poor quality secondary (stereo) speaker; third year with the same design; no headphone jack -- which didn't bother me but is a point of contention for some