2 GB of RAM is a huge difference regardless of your opinion of 3D Touch (which is a big feature for a lot of people, I'm sure). I prefer the S phones. They are much more polished generally.
I'm actually not seeing how they are the exact same phones. This is how the S line has been for years, and some people prefer to be on that track. Hardware issues sorted out, faster processor to keep up with major changes from previous generation (increased resolution), double the RAM, force touch. Seems like a big internal difference to me.
Isn't the 6s still running on a gig of ram? Anyway tiny improvements aren't worth the upgrade in my opinion. I have a Samsung Galaxy S4, and its good enough for me.