i have owned both an ortlieb and a chrome backbone. the ortlieb holds so much crap its ridiculous. the only reason i dont have it anymore is i gave it away. it had some holes in it from my pet rodents chewing on it, and i wanted to get something that had more pockets and stash room. plus it matched my friend's super-sweet pear izumi rain jacket, so how could i not.
the backbone is great. i think the ortlieb is more suited to carrying really huge loads, but it doesnt have the convenience of the extra pockets the chrome has (unless you spring for that little accessory they make). ive got a bulging disc in my back, so shoulder bags are out of the question for a while. the only advantage shoulder bags have over backpacks is the swing around factor. but if you tend to stuff your bag to the brim, like i do, then backpacks are IMHO the way to go. its easier to ride with them, they keep the weight centered, and when you pack them, you can assume things will stay generally where you put them while youre riding.

ps if i had the extra 100 bucks, i probably would have gotten a reload. but times being what they are...