The iPhone 6s display might come with one feature that the Galaxy S6 doesn't have though.
We've seen iPhone 6s rumors hint at Force Touch technology for awhile now and where there's smoke, there's typically a ton of fire. It also helps that iOS 9 has been confirmed to support the tech involved.
If you aren't familiar with Apple's Force Touch, it's technology that's able detect the difference between a tap and a hard press on a display. In turn, this opens up new actions that wouldn't be possible without it.
For instance, iPhone 6s users could push on the display harder to get access to a secondary menu item or action, add to in game controls or simplify access to some app options. It would be an interesting addition to the iPhone.
It's currently available on the new MacBook and the Apple Watch so if you have a chance to go hands-on with those two products, do so ahead of the iPhone 6s launch. That way you'll have a feel for a feature that could wind up being a key difference between the iPhone 6s and Galaxy S6.