It has improved speed everywhere ( new processor, faster wireless, quicker Touch ID sensor) ; a sturdier body; better front and rear cameras; and a bold new 3D Touch pressure-sensitive display which offers new iOS shortcuts.
The bad It has the same battery life as the iPhone 6. You’ll need to buy a larger iPhone 6S Plus or upgrade to an iPhone 7 for better battery life, plus optical image stabilization for photos and video.
The bottom line The iPhone 7 may be the new hot stuff, but - particularly with a price drop and a storage upgrade - the iPhone 6S is still a great deal.