The reduction in glass area probably gives some nominal reduction in weight, though probably no more than on the order of 10-20lbs or so. As for stiffness, that is far more influenced by the engineering quality that goes into either configuration rather than inherent benefits.
Any inherent difference between notchbacks and fastbacks (non-hatchback) are probably non-existant while hatchbacks are trickier to design to a said stiffness at a given weight, though it can be done with good engineering and perhaps strut tower braces as on, say, the 350Z, though that of course obviates some of the practical benefits of a hatchback.
Regardless, a notchback's upright design simply says staid "sedan" to me versus the fastback's far sleeker profile that has a far stronger performance personna to me.
