If you are in the cup holder mount camp (versus the vent mount or dash mount), you might have a problem with how low your iPhone 6s sits in your car's console. Most car cup holders are positioned in the center console at arms length, which is great for grabbing your soda, but not so great when you are trying to see which street to turn on next.
Cellet makes a Universal Cup Holder Mount with a rotating arm that is 10 inches tall. So, even if your cup holder is too far away, your iPhone 6s doesn't have to be.