I think this is an "all other things being equal" answer... and all those other things are never equal! The curved windway is a subtle influence that is easily dwarfed by other aspects of craftsmanship and design. My experience is that curved windways offer a bit more resistance and this gives something a bit more to "push against" with the air column. Very subtle, but it seems to be real.
Everyone says a curved windway gives a more complex sound, but I don't know this to be true. Acoustically, it's hard to understand how this would be an important influence, and we have many centuries of careful flue design in organ pipes... if curved windways were superior, I'd think we would see more of them in organ pipes. Perhaps the curved windway, if combined with a curved blade, would give more surface area for the blade (and the "wind reed") which could lead to more volume and presence... but I'm speculating.
I think this is a minor impact at best and probably shouldn't figure too strongly in your choice of instrument.