The 5S is a stronger design, but if you use an Otterbox style case it doesn't matter because you can toss those things across the room and they will be fine. That being said, the 5S is probably easier to hold for someone with small hands.
It's really up in the air at this point. Preliminary drop tests have been meh in my opinion. And no drop test accounts for a case. My wife has a 5s with an otterbox commuter and she drops her phone quite a bit (just did it yesterday in the garage buckling my daughter into the car). I've carried both a 5 and now a 6. I dropped my 5 on the concrete and got a few dings and about 1/4" little ding on the edge of the screen... nothing big.
I'd get the 6, personally... just because to me there is not a point in going back at all when it comes to technology. Get a 6 and an otterbox defender case or lifeproof or something and make him PAY when it breaks!!