Funny, I just bought a 6S a week ago. Battery life is poor, absolutely. The camera is no great shakes, but tolerable. Regarding screen size, honestly I'm torn... I don't know how to feel about it. On the one hand, I do like the fact that I can basically operate the phone with one hand (barely), as everything is within reach. On the other hand, the bigger screen real estate definitely has its perks, I won't lie. I do like the smaller size in my pocket. So... tossup? The battery life is the biggest negative, and when you go to low power mode the usability takes a nosedive (basically you can't read or do anything that requires the screen not to go black for more than like 30 seconds or something, and it goes black with no warning, so basically you're constantly turning it on... hate that. Yeah, the batter life is my biggest gripe. In fact it's so bad, that I'll take a serious look to see what they come up with in iphone 8 - if the battery life is improved substantially, I just might go for it.
If I were you, I wouldn't buy at this moment - I bought only because it was a really good deal through Virgin Mobile and it just made sense. But the 8 will be announced within days. At least wait to see how the two compare. Also, with the 8 out, the price of the 6S might drop. It's less than a week of wait, so if I were you, I'd wait.