I have both an open face & a full face. There have been ocassions when I've set off wearing one and carrying the other under the seat.
My open is a RJays Urban and I normally wear this for local riding in summer, and/or when I'm going to be riding Westward into the late afternoon sun. I even usually keep this helmet on when re-fuelling, and paying at the counter. That speeds-up the fuel-stop no end! This helmet is also my preferred choice when aboard my R100RS-powered outfit, and when I participate in the December Ipswich Toy Run.
I have a Nolan full face that provides very good coverage that totally eliminates wind-roar when the visor is pressed home. This is best helmet in pouring rain although it closes up so well that it can fogg if the visor isn't left not fully closed. Unfortunitely the full face isn't suitable for short trips in mid summer.
Sometimes I'll set off (Eastward) for Ipswich wearing the full face, with the Urban under the seat. That visor can't be beat for the return ride into the mid-to-late afternoon sun.