Recs.
Updated
SpecsUpdate
Pros
Pro Quick info on the home screen
While it doesn't add any extra functionality, 3D touch allows you to peek at apps for quick info, and perform tasks quicker than before. It almost acts like a right click on a computer. It's still relatively new, so third parties haven't all jumped on board yet. There is a lot of cool things that 3D touch can unlock such as a new level to gaming.
Cons
Con Limited customizability
While Android (and to a lesser extend Windows phone) let you customize virtually everything, the iPhones force you to jailbreak to get anywhere near the same functionality. Even taking control of the home screen needs use of folders for less frequently used apps, whereas on Android and Windows Phone the home screen is for frequently used apps and all the others can be left in the app tray.
Con Large footprint for screen size
The bezels on the iPhone 6S Plus are very large, making its dimensions larger than any other phone with a 5.5" screen. This makes it harder to reach the far corners with your thumb, heavier, and generally less comfortable to use.