Thanks to all for the range of well considered information. It appears from the comments that the gps screens are easier to see, which is surprising given that the screen technology is the same. Perhaps the gps has less detail and therefore easier to see?

I have used the Maps.me app and played a bit with Osmand, but I am used to google maps which can now be downloaded for offline use, so I don't see an obvious benefit for these other apps. Perhaps google maps does not allow uploading of pre-generated routes from computer?

Certainly important to have a robust mount, wouldn't care to see device being launched off the bike while moving down the road.