Wait for an iOS 9.1 jailbreak and customization problem solved. Had more fun with a jailbreak than I did with my rooted GS4.
I will say the Nexus 6P is the closest Android phone to an iOS experience though. Updates directly through google, no bloatware, great optimization because it's vanilla Android on common hardware, etc... Overall an extremely well rounded phone. The fingerprint sensor is as fast as the iPhone 6S's (iPhone takes slightly longer because of animations) and it doesn't suffer from Touchwiz's terrible RAM optimization, meaning all 3GB of RAM is usable. In real life usage I doubt you will notice much of a performance difference between the two, besides maybe loading speeds since the 6S Plus has insane flash storage. Have heard some complaints about the 6P's battery life, possibly an issue relative to individual usage rather than a universal one... just something to look out for.
I know most people on this website will probably give you a biased opinion, but honestly both are great phones, it just depends on whether you want iOS or Android. But the 6S Plus is an amazing phone and since you already own it, you'd be kinda silly to get rid of it.
IMO, if I were to have gone with an Android phone instead of my 6S, it would've been the Nexus 6P.