It's about double, plus you get 3D Touch and always-on Siri. It's up to the buyer but the only reason to get a 6 is if the screen size is more important than performance/longevity to you and you don't want to pay the money for the 6s.
Additionally, some iPhone 6 models were starting to get a defect causing the screen to stop responding. It definitely makes the most sense for Apple to discontinue.
Also, they've doubled the storage on the 6s so that should make it a better deal as well. The 6 would be the old capacities.