Below I am listing some of the top reasons why you should not switch from iphone 6 to iphone 6s. Though I am not discouraging you to buy iphone 6s if you are not already using an iphone 6.
1) They look the same : iphone 6 and 6s looks exactly the same and nobody can differentiate between the two phones just by looking at them. Although the new iphone 6s has 7000 series aluminium which is more durable. The bending issue was not a problem with iphone 6, so I guess it won't make much difference here.
2) Screen quality is the same: Both of the phone has the same screen resolution with 326 ppi pixel density.
3) Higher spec is unnecessary: The iphone 6 has the A8 processor which is already fast enough to run all the apps in app store smoothly. So, what are you going to do with a better processor? Yes the new A9 processor in the iphone 6s is faster than the A8 processor, but since it won't make much difference, you should rather wait for iphone 7.
4) 3D touch is cool but do you really need it ?
Apple calls it 3D touch, where microscopic sensors are embedded in several layers of the iphone screen. The sensors will receive pressure signal from your finger and give you extra input controls. For example, with one press you can open inbox to see the list of mails and again with more force you can open one of your mails. That means , you don't have to press twice , you can press only once but do 2 different things with different pressure applied from your finger. It might sound cool but my question is the same. Are you lazy enough to press it twice?
In the conclusion, I would like to say that , if you have not bought an iphone yet or using a very old model , you should buy the iphone 6s. But if you are already using an iphone 6, you should probably wait for the iphone 7 to come out. Because it's too early to switch and the difference between the phone is not huge.
I hope this will make sense. :)