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The S models are usually future proof.
6s+ has 2GB of RAM, the really powerful A9 and the 3D touch so if they'll implement new gestures involving force in future versions of iOS they will be supported.
Think about the 5s, it is still supported and iOS 9 runs smooth on it. You can expect the 6s+ to have similar performances in 2018
 
The S models are usually future proof.
6s+ has 2GB of RAM, the really powerful A9 and the 3D touch so if they'll implement new gestures involving force in future versions of iOS they will be supported.
Think about the 5s, it is still supported and iOS 9 runs smooth on it. You can expect the 6s+ to have similar performances in 2018
Technically 7 will also likely have at least 2 GB of RAM, and iPhone 5 supports iOS 9 fairly well too.
 
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