I think you need to consider what you want/don't want from a phone and what's important to you. As well, it may be important whether you have ever "switched" in the past (be it computer OSs, phone OSs, etc.).
My last iPhone was a 5S which I've now retired entirely in favor of Android phones. In my case iOS's lack of true onscreen widgets and weak notification system (at this point specifically the lack of a "dismiss all of this mess" option) finally became too much to deal with. But I've worked in both camps for years.
The 5S is a dated phone at this point--doesn't make it "useless" per se, but the 6P will be an upgrade (as would be an iPhone 6S/6S+). If the option is solely 5S vs. 6P then unless you really can't manage the change from iOS to Marshmallow the 6P should be the clearly superior device from a hardware perspective.
If you throw the iPhone 6S+ into the mix it gets more interesting. The phones are fairly comparable hardware wise *except* the 6S+ has a better camera. But, then again, the 6S+ is quite a bit more expensive. (64GB 6S+ is $849, while a 64GB 6P is $499)
There will be a learning curve if your only experience with a Phone OS is iOS and expect some frustration up front. But the same is true for an Android user going to iOS (as I said, the lack of widgets simply became a killer for me, but your mileage may vary).
One advantage of the 6P is that, being a Nexus phone, the whole "Android phones don't get updates timely" question goes away (aside from Google's rolling update that may push you a little bit back based on the luck of the draw or lack thereof--but of course that would generally avoid most of that little problem Apple hit on the first iOS 8 update).
The fingerprint sensor will be quite an upgrade from that on the 5S, in line with that on the iPhone 6S/6S+. That said, I find I really strongly prefer the fingerprint sensor on the back and not "hardwired" to a home button that wants to do other things when you press it. But if you put your phone in a stand and want to unlock it while docked (which I don't) then you might prefer a front mounted sensor. But having it on the back is way more convenient when you are out and about with the phone.
iOS still tends to get applications first and, as well, in virtually all cases you'll need to buy a new copy of any purchased software even if the version exists on both platforms. And some things simply aren't allowed under iOS (though recent changes to the OS have allowed some of the more significant limitations to be less troubling--such as extensions).
That said, there's no such thing as a perfect phone for everyone, regardless of what the reviews (or various company's marketing departments) may tell you. If you determine what's important to you and start going down that path I think you'll figure it out quickly.
The fact I find the 6P perfect for me does make you somehow "crazy" if you decide that the 6S+ is your phone. And the converse is true as well--while a fine phone, I'm not crazy for finding it deficient for my uses.
Facebook
Twitter
Instagram