I haven't used 6s but used iPhone 6 which had a mediocre battery life to say the least.
I needed a recharge by finish of working hours by 6pm almost every day, expect a 2 to 3hrs SOT at the max. (don't go by reviews saying 10hrs video etc because real life scenario you mostly multitask). Its same with iPhone 5s, in fact I've seen most of the iPhone 6/6s users always carrying a powerbank or using a ugly bulky battery case. Thats why I was suggesting you to buy IPhone 6s plus instead, its anyday better (plus better camera) than ugly battery case or power bank.
IOS and Android 6 have matured to certain extent that the relative advantages of either over other are subtle and subjective but not universal. Let's leave it at that.
The hardware advantages of Android flagships are currently quite decisive over current iOS flagship by a huge margin.
I'm currently using S7 edge which lasts for 1.5d easily, if I compare the hardware advantages apart from battery life,
Dual sim
Better Camera
Extended Storage
Quick Charge (takes half time to charge)
Wireless charging
USB drive connectivity (OTG support)
SAMOLED screen with qhd resolution
I'm not counting faster Chipset, Ram etc
Aesthetically equally good looking (though looks are subjective, but iPhone 6 is the ugliest iPhone ever IMHO)
These hardware features make you feel iPhone 6s is lagging by 2yrs in development, which is quite visible in iPhone sales & Apple stocks depreciation.
Currently other than your affinity to iOS there's no single reason to buy iPhone 6s over Flagship Android. Since you aren't a previous iOS user, I'd suggest you to buy S7 or comparable flagship Android.
Wait for iPhone 7 if you want a even comparison with current flagships and decide.