I'd say the battery life is decent/ok on the 6S. I get through my normal day, typically without a bump charge, just fine, and I rarely need to use low power mode. I'm a moderate use person, though, so ymmv. I have a charger at work and in my car just in case, but I rarely need to use them. If you're a heavy user, strongly consider the plus.
Compared to the 6S+ the battery life is substantially less, probably 25 to 30% less or so. For me the size, ability to use with one hand, and run and workout withthe 6S trumps the battery life, OIS, and screen quality of the 6S+.
As an aside, and in my experience so ymmv, I've never understood the laser focus on battery life. But it’s not because the Plus has made it a non-issue, it’s just that battery life has never been an issue for me with any iPhone I’ve owned. Honestly I can’t remember the last time I had an iPhone that I had to regularly keep charged throughout the day. I know people who say their iPhone has a dead battery by lunchtime, but I just don’t have a grid for that. So, the advantage of the better battery life of the 6S+ is (unfortunately?) wasted on me.