Depends on your needs in a phone, cash on hand, etc. I have a bit of a different view from some of the others. If you are talking 6 to 6S Plus, I would back that since the change in size alone would be quite a difference.
From the 6 to the 6S? I couldn't recommend that to anyone without a guilty conscience unless they upfront suggest they have money to burn or are annual upgraders. The 6S is fast, but so was the 6. The 6S camera is nice, but given they upped it to 12mp without focusing on the sensors and aperture....I don't see much if any photo improvement. I find 3D Touch and Live Photos to be gimmicks. Visually....unless you get rose gold they are identical aside from a little heft due to reinforcing the frame. Still mediocre battery. Still no OIS. Still 326 ppi.
As always, this is merely my opinion and nothing more. But for me, the 6S seemed to be Apple's least effort yet. I cannot put into words how unimpressed I was with iOS 9, and then to combine that with essentially an iPhone 6.1? I just am not sure what the powers at be were doing the past year.