How you carry the bag makes a big difference. I used a laptop bag once and was horrified how heavy it seemed, compared to the same weight in a rucksack.

A rucksack can also be the bottom of an improvised bivvy bag, if you are wearing a long jacket, is a lot more expandable; you can tie items to it and one can easily carry a coat under the top (ideally get longer staps fitted).

Also of course a laptop bag screams: 'something here worth stealing'.
qjs