iPhone 6s

The difference between iPhone 6 & iPhone 6s are minimal, the S upgrades are always hardware upgrades (exempt for iPhone 5s, for Touch ID). E.G.

• iPhone 3G to 3GS - Speed Upgrade
  
• iPhone 4 to 4S - Speed & Siri Upgrade
  
• iPhone 5 to 5s - Speed Upgrade (meh Touch ID)
  
• iPhone 6 to 6s - Speed, New Rear Casing (Aluminum 7000 Series, to fix #BendGate)
  

As you can see the S are just Speed Upgrades & adding 1 or 2 new features such as iPhone 4s Siri & iPhone 6s 3D Touch.

I personally hate the iPhone 6 & 6s due to the stupid design & the antenna lines at that back with the stupid camera sticking out & the phone slippery as fuck (fix is possible with a case) and this is coming from a AppleFanBoy.

I would say if you have a iPhone 6, no point to upgrade to 6s, waste of your hard earned cash, same from iPhone 5s to 6s. I would personally suggest to wait for the iPhone 7, reasoning being:

• New Design
  
• Will be faster then the 6s
  
• It will have 3D Touch, it wont go anywhere for the iPhone 7
  
• Might fix chipgate (search it up if you don't know what it is)
  
• New major features (like TouchID & 3D Touch & More!)
  

Again my suggestion is wait for the iPhone 7, if not go with a Samsung Galaxy S6/Edge or Note 3/4 for now.