According to several iPhone 6s leaks, the iPhone 6s design will be made from a new 7000 series aluminum. This is the same material that Apple uses with the Apple Watch Sport. This change is in response to the Bendgate controversy from last year. Bendgate outed a weak spot in the iPhone 6's metal design.
This change is exciting because it will give consumers peace of mind. You won't have to worry about the iPhone 6s design bending or getting damaged. 7000 series aluminum is extremely strong and durable and those are qualities you want from a smartphone design.