i'm not really coming from the S6 or N5, but have both platforms coinciding with each other. well, an S4, N3, S6e, N5, 5s and now 6s+. but the N5 and 6s+ are my main two...
there's a lot to like about the 6s+, but there's just much more i love about the N5. a lot of my points will be between either the 6s+ and/or iOS or the N5 and/or Android. i like how every app i download, it gives me the option to allow the app access to whatever other app right away rather than the Android way where you have to accept before you can even download to install. basically, i can control each app as i go. i also love how there are many of apps that integrate Touch ID when accessing them. even Google apps. i don't understand why Google hasn't allowed this for the S6e or N5 on their own Android platform.
it still bugs me how much slower (or more steps it takes to do something) on the 6s+ than it is on the N5. so regardless of hardware specs (or how much more optimised iOS is with their hardware), the nature of operation on iOS can bug the heck out of me from time to time. this is especially an annoyance when i'm multitasking with at least several apps and are deep into them. and even after iOS 9 bringing multiwindows, it's still not available to the Pluses???
notifications could be better for iOS, and the control center can definitely be much better like allowing it to be customisable. argh!
Android is a heavier OS copared to iOS especially if you like to customise. so, that can drag things down depending on. on how heavy you customise things especially with a Maker's added UI skin. i tend to keep my customisations minimal. i personally run Package Disabler Pro (disables packages from running in the background) alongside Greenify (hibernates apps you don't need running in the background till you actually use them) all without having to root. this helps with fluidity of navigating around the OS and of course with ScreenOn Times. even with all that done, i think my 6s+ SOTs is as at least equal to my N5's if not better. probably better. it's only been 3 days and i've mostly been playing with the 6s+.
screen quality goes to the N5 all the way. it's night and day in comparison. sometimes i feel like i'm looking at the quality of a thumbnail version of an image (not thumbnail size, but quality). basically, it's just not as sharp and detailed as what you can see on the N5 even with the bump in PPI with the new 6s+.
the speaker sounds better on the N5, but the audio quality from the headphone jack on the N5 is amazingly awesome! that Wolfson digital audio converter hardware it's using makes even the plain old included earbud set sound like they were a bigger headphone set with bigger drivers. bring your earbuds to a store and just listen for yourself.
for connectivity, i seem to have faster download speeds on the N5 over LTE, but the 6s+ definitely has much faster speeds over Wi-Fi.
there's a bunch of niggles, but those are the ones i remembered to rant about. haha