I can tell you this. I just picked up a 5s for my daughter. I have a 6s Plus. Just for fun I did a comparison with opening apps and switching between apps. The 5s still holds its own. Most were just as quick to open and switch between. Obviously things like games are going to be better on the newer 6s but the day to day stuff still works great on a 5s. I have not used a Galaxy s6 but I think if you are ok with the 4" screen that you will be fine. Plus you can pick up a 5s for a good price. Good luck.