I personally think the 6s is significantly more heavy than the 6. I didn't even have a case on it and knew it. I also think in a day or two I will not even notice so it's not a complaint. I do not like the weight but it's not a deal breaker since it is only slightly heavier. I think the biggest thing about the weight is the balance used with 1 hand and reaching the top of the screen.
Again, not a deal breaker, just noticeable.
I can't seem to find anything on a problem I'm having with my 6s though...when the phone is sleeping (or I don't use it for about 5-10 minutes), it hangs for about 10 seconds to unlock or start the app I click on (whether it's messages, email, Safari...). Not to hijack the thread, just curious if anyone else has noticed this.