For hardware it's pretty much a wash, both are exquisite phones at the top of their game here with a few notable differences -
6S+ has OIS, 6p does not. For still photos both are on even ground, but the OIS will allow for more stable video recording on the move.
- The 6p has an edge on the 6S+ for display resolution
- The speakers on the 6p are louder
- The 6p uses android on screen buttons, the 6S+ uses the same physical home button iPhones always have. I prefer the former personally.
- The 6p charges faster
On to software, this is where things really differ in many respects because we're obviously talking about 2 different OS's here and your preference is going to be the deciding factor. The one thing both share in common is that neither phone holds an advantage over the other for a fast and smooth experience. But there are several key points about iOS that I dislike -
- Sharing data between apps on iOS is still atrocious. This is old news though, you've been allowed to share just about anything to any app on Android for ages, while Apple has staunchly refused to budge on this for just as long.
- Similar to the above, there's no control of file management on iOS. I can't stand that. Everything must be managed according to the apps that control those 'files' and/or through iTunes. This is such a needlessly outdated way of managing data.
- I prefer Android notification handling and overall notification pane layout.
- Google Now remains better than Siri. I really expected that this was the year Apple would pull a rabbit out of the hat with a surprise Siri overhaul but they didn't. They did add some contextual awareness improvements but not enough to fully bridge the gap between the two.
- Still no freedom to set default apps in iOS
- 3D touch after the initial hype wore off turned out to be kind of underwhelming. I see this feature as something that's in its transition phase. Now were Apple to really focus on it in the future and use it to drive an entirely new method of navigating the iOS UI by getting rid of the physical home button that would be pretty groundbreaking for them, but that's not now.
- Unfortunately for Apple, their efforts to close the feature disparity in the last few versions has also ensured that iPhones now have just as much of a share of their own bug issues to deal with now
tl;dr version: Both are at the top of the smartphone food chain and my above points are all fairly techie reasons which most buyers will won't care about whatsoever. I'm simply presenting my POV from a techie standpoint because of where we're having this discussion. If I were to mention most of my reasons for preferring the 6p to a friend or coworker who never sets foot on sites like this (and I have) I'm usually met by a shrug, blank stare and a "yeah so?" response