I think you have to be realistic about HOW LONG you will be carrying a laptop on your shoulder before you take a break. 10 minutes? Half an hour? 3 hours? Realistically? A 'standard' notebook is no problem to carry in a shoulder strap bag for up to an hour at a time I'd say. Any more (like my recent siteseeing trip around London and you'll wish you had an Air. I certainly did! Most people go for only a relatively short distance before sitting.