In the last year or so, new aero helmets such as the Giro Synthe ($250) and the Kask Protone ($300) have ushered in a new era in road helmet design. These helmets are more rounded-off at the tail, and more aero overall than previous top road helmets such as Giro's own Aeon and Specialized's Prevail.
In addition, unlike Giro's older Air Attack, the newest aero helmets such as the Synthe -also- ventilate well. This is a paradox that is hard to pull off.
As for aerodynamics/quietness, if you compare the Synthe to previous top of the line, all-around road helmets (Prevail, Aeon and before that, the Giro Ionos), helmets I rode in for years, there is no comparison. The Synthe is much quieter, especially when you put your head down during hammertime. The shaping at the tail of the old school helmets (Prevail/Aeon/Atmos/Ionos) is now obsolete.
The new Giro Synthe and Kask Protone helmets are "game changers."
Lastly, the ventilation of the Giro Synthe is different (and very effective). You actually feel the cool air running over your head -inside- the helmet (through the internal channels). This is brilliant. Does it vent *quite* as well as the Aeon or the Atmos? I dunno. We'll see this Summer. [UPDATE: done]. So, I do use my old white "summer" Aeon or Atmos helmet on rides > 90°. But in all other conditions, I much prefer the Synthe.
Versus the Kask Protone, I like the looks of the Synthe more (totally personal). And the Synthe is less expensive.
By the way... Greg van Avermat and Anna van Breggen just won the 2016 Olympic road race in Rio. They were both wearing the Giro Synthe helmet.