The 6S Plus is definitely the best option out there with its gorgeous screen. But the 6S still holds an appeal to a massive number that don't want to go bigger as they simply can't handle anything larger... (insert inappropriate joke here!)
I'm loving my 6S Plus, even 3D Touch which I don't use that often but sometimes for convenience. The only issue I have is with the Music app not supporting a landscape mode. I honestly think that Apple shouldn't have any issues with enabling the iPad version of the Music App on the iPhone 6S Plus as it's screen size and resolution should be able to handle that perfectly.