I have experience with 3 aero helmets: Giro Air Attack, LG Course and Specialized Evade.
I got the Air Attack when it first came out and rode that helmet right up til the LG Course tested faster (on most riders at ERO Sports). Sold the Air Attack cheap to a teammate. Never had any real issues with the Air Attack, even in pretty hot weather. Mine was matte black by the way.
When the Evade was introduced and tested faster than the Course (on most riders at ERO Sports), I bought the Evade but this time kept my old helmet, the Course. I did this for two reasons (1) I thought the Evade was a little "much" when doing stuff like cyclocross or easy recovery rides and (2) I was concerned the Evade might run hotter than the Course and I would do well to keep the Course for hot days.
I have lots and lots of miles on both helmets and can say that for me, the heat concerns with the Evade were unfounded. I don't notice any cooling difference between the two helmets, even on hot climbing days, including this ride (http://app.strava.com/activities/194119537/analysis
) of 145mi with >13K ft of climbing with temperatures above 110 degrees! Yes, that day was hot, but not any moreso because of the Evade.
The two differences I have found between the Course and Evade are (1) The Course favors a rounder head shape while the Evade favors a more oval head shape (my head is a bit more oval so the Evade happens to fit me better than the Course) and (2) I can't stash my sunglass arms in the Evade the way I can with the Course. The vents on the Evade just aren't placed right for this. On the Course, like with most helmets with lots of vents, I have a couple options for stashing the sunglasses "in" the helmet.
With that said, if the Synthe tests faster than the Evade, I will most likely switch once again, so long as it its well on me. I do prefer the look of the Synthe to the Evade, but I have grown to appreciate, if not completely love, the look of the Evade. YMMV.