Positioning of mobile phone on handlebar
I use osmand for navigation and much prefer the portrait orientation. Can’t really say why since I always have the map aligned northwards, so it’s not about being able to look farther ahead. I guess it’s just because I’m used to it and all apps work in portrait mode but some do not work in landscape mode.

The Topeak Ridecase Mount (+Omni Ridecase silicon straps to attach the phone) I use can easily be turned 180° though. I can highly recommend osmand and the Topeak mount.