For incredibly large boxes, I would insist on a front rack similar in style to the CETMAs, but for things that aren't so bloody heavy, it seems like a backpack (and maybe a hiking version) would be a good solution to centralized pressure on one shoulder. As for seeing around a bag in traffic, I would imagine that hiking backpacks would be more difficult to see around due to their height. Sure they're better stabilized and pack nicer, but if you're more sensitive to being able to see over your shoulder in exchange for taking a couple minutes to find something at the bottom of the bag, I'd say a messenger backpack with some nice stability would be more appropriate. If we're talking about daypacks, I suppose that's a whole different matter.

Then again, now that I compare hiking and messenger backpacks a little more, the width is what matters. I've never worn a gigantic backpack in traffic, so I guess this is all supposition anyway.