While I'm planning to get the 6S+, limiting it to one model means developers will be really slow at adding it to their apps (if at all) and that's not good for anyone
THIS. As a 6 Plus owner, many apps still haven't added any extra support for landscape that makes the capability worthwhile. Even as a 6 Plus owner, I'm not even adding that capability to an app that I'm lead design/project manager for right now. The money to add something like that and test it just isn't worth it at this point, and honestly in our case I'm not sure it would add much benefit. From what I understand it seems to be more trivial if you already have a universal iPad version, which we don't.
Force touch or not, no way I'm staying with the Plus. I regret trying to stick with it past my return period. It's just too unwieldy, and the software on it has been extremely buggy for me for whatever reason (I've seen very mixed comments about this, but I seem to be in the minority with the various issues). I cannot wait to go down in size. I did want something bigger than 4", and I think 4.7" will be better. I'd prefer 5" with a reduced bezel, but whatever. I do really like the screen on the 6 Plus, so I hope they can put an @3X display in the 6S as wellalthough that is unlikely.
The thing is that the 6 Plus already has lots of advantagesadvantages that are not likely to come to the 6S: OIS is too bulky to fit into the 6S unless they make the phone thicker or have developed thinner modules since then (anyone know?). Then you have the ridiculous battery life which can't be easily matched just based on the size difference alone. Lastly you also have a sharper display, which could come to the 6S but I think they might just wait until the iPhone 7 to do true @3X on the Plus (right now it's scaled down) and on the regular 7.
So you would think that Force Touch would come to both sizes since they already have some key differences. Apple would want developers to quickly adopt the new interface interactions, especially if it could be considered a key advantage over other Android phones. The only thing that makes me wonder if the Plus only would get Force Touch is if Force Touch is the key to split-screen multitasking. People have always wondered how Apple would do that on the iPad and I think something like a Force Touch pinch-out gesture could work well. But would Apple really put split-screen multitasking on the Plus? It seems like that would be really cramped. But again, that's the only functional reason I can think of for only adding Force Touch on the Plus.