According to the official environmental documents from Apple, the new iPhone 6s and iPhone 6s plus is around 11% heavier. And that is the only thing I don't like.
The new display assembly weighs twice as much as the display assembly used in last year’s iPhone models. In the iPhone 6, the display assembly weighed 12 grams. In the iPhone 6s, the display assembly weighs 24 grams, more than double the weight of last year’s model.
Overall, however, this year’s iPhone 6s and 6s Plus weigh 14 grams and 20 grams more than their predecessors, respectively. The switch to 7000 series alloy aluminum accounts for just 2 grams of the weight increase of the iPhone 6s Plus, while the iPhone 6s’s aluminum actually weighs a gram less.
Personally I don't like that it gets heavier. I want my devices as thin and light as possible. It gets a lot better user experience then. Especially if you use them for work as I do. If you create content on the device and hold it in your hands for a quite long time, it's necessary that it's as light as possible.
That's why I love my iPad Air 2 so much. The thinness and lightness really make a difference to the iPad Air 1 if you use it as I do.
Of course I will still buy the new iPhone 6s plus, and I'm sure the benefits with the