I’ve been wearing a Giro Synthe MIPS for a few years (actually a couple of them. They have a great crash replacement policy), and I’m a big fan. Good balance between air flow and ventilation.
A few weeks ago, I picked up a Giro Vanquish MIPS, and I think the airflow and aero performance may be slightly better than the Synth. I like the visor, and will certainly wear it in TTs, but it’s a bit extreme looking for every day or crit use. Aero vs peer pressure - tough call!
FWIW, I always wear a backwards wicking cap under my helmet - I sweat like a stuck hog, and I train in the swamp, so heat and humidity are omnipresent May through October. The Synth has performed well in the heat. I look forward to seenig how he vanquish does this summer.