I've travelled a fair bit using a a messenger bag for a few weeks a couple of times, always very annoying and wouldn't recomend it, they are best for cyling not for walking around.
A backpack set up correctly should rest on your hips with its weight so not much pressure on your collar bone, I've dislocated mine a few times and its always been ok with backpacks. Take as little as possible and a 30 litre bag should do you fine.
I've travelled a fair bit using a a messenger bag for a few weeks a couple of times, always very annoying and wouldn't recomend it, they are best for cyling not for walking around.
A backpack set up correctly should rest on your hips with its weight so not much pressure on your collar bone, I've dislocated mine a few times and its always been ok with backpacks. Take as little as possible and a 30 litre bag should do you fine.