i have an ortlieb (don't know the model) and a freight backpack. I wouldn't recommend the freight to you. it's an amazing bag, so amazing that you probably won't use half of it (nor will I, although I'm trying). But, it's expensive, and huge, and really cushions around your back (to prevent it from moving around) which leads to a sweaty, sweaty back.
the ortlieb, on the other hand, has a foam "spine" on the back: two rows of cushions that suspend the bag about a half inch off your back, providing nice ventilation. true, the ortlieb is a one-pocket design, but mine came with a removable pouch to organize some smaller things, keys and pens and the like. other than that, it's a pretty narrow tube that rolls down at the top. i was put off by its uber-simple design at first, but its simplicity is the best part about it. if you need to, you can organze your stuff with smaller bag inserts (you can find see-through mesh bags used to organize camping gear). the best part about ortlieb, however, is that they are completely waterproof. not a drop of rain is going to get in your bag, and the bag itself is going to last years and years. well worth the hundred bucks i spent on it.
remember: you get what you pay for. err on the larger side and look for compression straps to reduce bulk when you're not using it.