Both brands are premium helmets that will perform equally well in a crash. Provided they fit properly. I've had several different brands and so far a Shoei is my favorite. All the helmets I've had have been medium.
I have a Shoei X11 now. It fits my head comfortably and is snug. The visor has clicks to stop it at various points in its movement. On the left side there is a tab that can lock it closed, open it slighly to vent condensation, or set to be unlocked. It is light and has good ventilation. The liners are removable for cleaning. It has a nose guard to keep breath off the visor.
The Arai Quantum F was a great helmet, but was tight in the jaw and forehead for me -- Arai describes this helmet as being made for people with a more round shaped head. Its visor does not have click stops through its movement. Instead it has a more constant resistance. It also has a lock that can hold it closed or partially open -- though this lock is on the lower left edge of the visor. The chin has a small pull down spoiler that worked to keep cold air off my chin and neck. Again it is light
The KBC VR1 has click stops in its visor movement like the Shoei, but no lock. The lack of a visor lock resulted in the visor sometimes opening when I turned my head sideways to the wind (to check a blind spot.) There was no way to open the visor slightly. This helmet tended to buffet on me and fit much looser than the other brands, especially along the jaw. Its medium was more like an Shoei large.
The AGV helmet was the heaviest of the bunch. It had a visor like the KBC, but tended to stay shut better. Again, no lock or way to pop the visor slightly. The padding was dense and didn't seem to mold to the shape of my head very well. The venting was poor and it had no nose piece to keep breath off the visor. This helmet tended to fog easily.
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