This; for a camera lens, which is generally recessed anyway, sapphire makes a lot of sense as you really want to protect it from things like sand, dirt or keys or whatever that can get into the gap and scratch it (and ruin all future images). The chances of actually getting something into the camera lens with enough force to break it is pretty slim.
The screen meanwhile is a big area that's begging for trouble if dropped, and is a bigger area as well, so the less expensive, more impact resistant material is the better fit.
They're really two very different products for different purposes. Now if the scratch resistance of gorilla glass can be improved even further then that'd be great, as dropping a iPhone still risks scratching it.
I mean, really there's no point in testing these materials to breaking point for tablets and phones to see which is better; what's important is whether they're good enough to survive what they're intended for, which is either typically drops (e.g - from pocket or handheld height) or from scratches that impair usability (which is pretty much any scratch when talking about a camera).
