The front replacement glass dimensions (width and length) for the iPhone 6 and the iPhone 6S are the same. The glass for the 6/6S will both fit into a iPhone 6 LOCA/OCA refurbishment mold.
What to watch out for:
The thickness of the aftermarket glass varies from vendor to vendor. The home button holes may be smaller not allowing proper fit of the home button. In the early days of the iPhone 6 glass coming out some where a little longer and would not fit into the mold. Some of the 6S glass my be a little longer from time to time depending if the manufacturer has got their tooling right.
The force touch is controlled by a sensor behind the back light. The glass has nothing to do with the force touch function. Though I imagine a thicker glass will have a slightly different feel(the type LOCA/OCA does make a difference on force touch).
Dry fit your aftermarket screens into the mold after you receive them from your vendor. You can do this with the plastic covering on. Because you need that plastic covering on to keep the glass super clean before laying that glass down, especially the inside facing the LCD. Have a stock OEM home button to check the home button holes (just stick it in) or buy a dial caliper and measure. Why? Because let's say everything goes right and you are getting ready to put the screen on a phone and the home button doesn't fit, all that time and effort is wasted. Been there!
That's all I can think of right now. Feel free to ask any question you might have.