for larger loads, i would much prefer a backpack - one that is designed for couriers (that is, really ****ing big). when i have a messenger bag on all day, lugging **** around, i develop pain on my bag shoulder. it doesn't even necessarily require that there be particularly heavy loads. it's the simple fact that it's a bag that is only over one shoulder. i also find that this affects my posture. with a proper backpack, too, you are more able to keep packages flush against your back without having corners and **** jabbing into your kidneys or lodging under your scapula or whatnot. carrying a file box in a messenger sling bag is totally ****ing awful when compared to one in a backpack. using padding or "a buffer" can mitigate this problem, of course, but i find that messenger bags suffer from it more than backpacks - simply as a matter inherent to their designs. in any case, i think the weight distribution issue really seals the deal.

if you like the ability to get into the bag without totally removing it, you can do pretty well with a backpack by making it so that one of the straps can be unhooked so that you can swing the bag around. it's not quite as effective as just sliding a messenger sling bag around, but it's not too shabby.

the only real advantage of a messenger bag is that you can get into it while it's on, but, really, if you have a large package, you aren't going to be able to get it out without either loosening the strap to the point where the bag might as well be off or just taking off the bag.