I get to inherit my wife's iphone every 2 years when we buy a new "S" model for her; so I started with a 3GS, to a 4S, to a 5S last fall when we got a 6S for my wife. I would not call the 5S's tech specs "outdated": the only feature it could use to future-proof it is Apple Pay. The 5S was a significant upgrade from the 4S: the fingerprint sensor is gold, the extra screen size is nice to have (but still easily used in one hand), and in general the phone is much more responsive.