I think Apple is primed for a great 6S/6S+ show.
Think of it this way. How was the 6/6+? Great devices, but equally great headaches, right? I mean bricked devices, iOS bugs, bending, etc really hampered the 6/6+ from being seen as favorable as it could've been.
The S is generally for under the hood, software, and sweet features that are unexpected. I expect a strong iOS 9 with more useful features that Android may offer or some that Apple feels is necessary for the core iOS experience.
As for the hardware, 2 GB of RAM on a 1.7-2.0 GHz tri-core processor paired with iOS optimization should make the experience even better. A new 12 MP camera with OIS on both devices plus a DSLR experience has me excited. Expect a much better front facing camera since the Note 4, G4, and S6/S6 Edge boast good front facing shooters.
I really want the force touch gestures to be very good and imperative to the core experience to allow alternative methods to different functions.
The rest of the features are unknown at this point. Apple doesn't play "catchup" as quick, so they focus on what they can marginally improve and then go for unknowns. The best thing to do is speculate from their mergers, acquisitions, and recent patent history.
I mean the S line has been good to iOS users and it took me awhile to truly appreciate what Apple does because I used to be totally pro Android and anti-Apple. The iPod Touch and iPad really changed my mind.
4S: Siri, 5S: Touch ID and 64 Bit Processing. 6S? Unknown.
Siri led Google to unleash Google Now. Microsoft lagged behind, but released Cortana. That forced the mobile OS' creators to step up their game.
Touch ID: People are into the idea of using fingerprint biometrics to protect their phones. I have been using a fingerprint scanner since the S5 and I cannot enjoy other phones as much if they lack the feature. Now with Apple Pay, Samsung even made their Samsung Pay for later this yar.
64 Bit Processing is kinda unnecessary, but interesting. It can help mobile gaming out with coding languages like Metal and provide some powerful SoCs or GPU/CPU combos for high end mobile gaming. Frostbite on iOS 8 was pretty interesting last year. Wait till it becomes the standard with 4 GB of RAM? Android is closer to that, but I think Apple will make the most user friendly SoC and RAM combo given their experience to understand customers' wants.
As for the battery, 6S+ will be the best, but I hope the original 6S will have something to avoid having to charge frequently. Wireless charging and quick charge are musts now.
Innovation, even in small aspects, on both sides of the aisle is health and key to future mobile development. I like the back and forth between the Galaxy and iPhone now. I use both the 5C and S6 Edge, but I continue to use my S6 as the daily driver. iOS is great in spurts.