I prefer a backpack to a messenger bag, generally. The most important reason is that a properly designed and fitted backpack will not shift weight when scrambling or climbing and will generally slip through thick brush more easily than something carried sidesaddle.

On the other hand, it is perfectly possible to sling a pack on one shoulder when you will only carry it a short distance and then take it off. Basically, a backpack is more versatile and will carry a given weight more comfortably.

Agree. My EDC backpack is normally carried light and on just one shoulder. Loaded up for a more serious walk and both shoulders and a sternum strap and waist belt make it quite secure.