i don't think any of the options could really be called a 'big' mistake. they all carry stuff, so that's the prime directive covered right there.

i've used backpacks, which were fine. now i've got a sort of crypto-messenger thing i got as an experiment and i wouldn't go back. i expect to be graduating to a real messenger bag when this one bites it. no stabilizer straps and it's basically rectangular, but even with those factors i don't find any comparison. i think it does depend a lot on what you personally want from whatever you use, but here's my list:

- access. someone else said this already. it's easier to reach into a shoulder bag, is one reason. i never found outside pockets saved me from having to get out of the damned backpack, especially when i wanted something that wasn't in one of the pockets within 'easy' reach.

- how they organise your stuff. i don't carry a lot of weight, but i carry a lot of different things; pretty much most of what i might need for most of my life through the course of a full day, so i'm probably in there a good 15-20 times a day. in my backpack that stuff was always layered in there, and there wasn't any pattern to what i'd want at a given time. i spent more time unpacking it from top to bottom, or opening every single sub-compartment . . . my shoulder bag is wider than it's deep, and has a single zipper right across the top. i still rummage, but it's much less severe. i never found the elaborate backpacks with all the different compartments helped with this in the least - in fact they seemed to make it worse. someone more organised might get different mileage.

- they've both been stable, but i prefer the shoulder setup. i find that even without a lateral stabilizer, it works better on this factor than my backpack did. weight doesn't affect that, unless it's in a positive direction. backpack was okay too, but if i had two gallons of milk in there things could get pretty weird pretty fast any time i swivelled a bit to do a shoulder check.

- fast transfer. i really really like being able to just get off my bike and start walking as soon as the lock's put in place. for me, the cost of riding with my stuff on my back is a tiny inconvenience compared with the inconvenience of having to fiddle around with panniers or anything else.