iOS 10 works just fine on the 5s. It is buttery smooth on that device. I can confirm that just playing around with my gf's 5s on iOS 10.
6 is about same in performance as 5s so you should be safe with upgrading.
Further anecdote -> I used a 6 abroad last week on 10.2.1 and it ran perfectly.
Up to you though.
I will echo sentiments that iOS 10 runs very nicely. Both iOS 9 and 10 were/are miles, miles better than iOS 8. That was a mess. But I digress - point is OP, going to iOS 10 should be fine.
And 10.3 is even better to jump to.
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DO NOT UPDATE. I REGRET IT EVERY DAY ON MY 6S+. WISH I COULD GO BACK HONESTLY.
10 Offers little to no improvement for features. Just changes some things around and make it harder to use old features. Especially the notification/messages. What a pain... Plus I'm sure performance will take a hit. As a general rule, stay on the same iOS version your device ships with. That's what it was made for.
As far as performance goes, the 6s plus should be more than powerful enough to give you a buttery smooth experience.
Judging by my 6s, opening every app from games to loading safari to messages to anything, it is *identical* and indistinguishable from my 7 in performance. Identical. Might as well be 2 iPhone 7's running the OS. The one sole difference is the physical button press on the 6s delays the response to go home vs the 7's faux home button, but if you use 3D touch or touch id with touch to open enabled, they are absolutely identical.
iOS 10 is super smooth and it's the first time Apple has released an OS on a full range of capable devices.
They have also really smoothed out the bugs that have been persistent since iOS 8.
I'm not on 10.3 yet (downloading right now) but it is supposedly even a wee bit more efficient with slightly quicker animations.
The only thing I can think of, which is a fair criticism, is pressing the home button rather than swiping to open the phone. On older devices without touch id (so you cant change the option to just touch to open) such as the 5 and 5c, this would be annoying.
But if we're talking about performance, it runs remarkably well on the 5 and 5c, both those devices are much less powerful than the 6s plus.