I have a iPhone 5s 16 gb & my brother has an iPhone 5 16 gb we are trying to figure out what iPhone would be worth buying between an iPhone 6 64gb or an iPhone 6s 16gb ?
There's a few variables. Some people manage to just deal with 16gb, something I can't see myself doing but if it really came down to it I'd choose the new hardware. Isn't the difference between 6S 16gb and 64gb about $100? Worth spending that extra amount for more space, especially with 4k video.
I would not buy a 6. With the front camera moving every 3 weeks, the screen clicking in the lower right corner, and bending issues, it's just not a good phone.
I've had every iPhone since 2007 and the 6 was the worst one yet. With the 6S they have stronger aluminum for the bending issue and a new screen and digitizer that should fix the camera moving and clicking.
If money is an issue, I would get the 16GB 6s. Just be aware of storage limitations, but you've evidently dealt with that with the 5s. If you can come up with another $100, I think you would be very happy with the 64GB 6s.
The 6 is not a bad phone, but the 6s is so much better.