I wouldn't be surprised if real-world tests confirm that the iPhone 6s and 6s+ show that their battery life is exactly the same as the current versions. I'm sure Apple will advertise the usual extra-hour of battery life in 3G, LTE, Wi-Fi, etc., but that will probably be due to the extra-hour they already told us about from iOS 9. Since the new batteries are smaller and since the iPhone 6 and 6+ were also advertised to get that extra-hour of battery, their battery life will probably be very similar.
I hope the A9's efficiency more than makes-up for the smaller battery (which no matter how you put it, people will be upset that it wasn't larger than before, even 1mAh).