A few weeks ago I asked about the relative protection of the Brain Cell vs. a Booq Viper. You should be able to find the thread if you search. The consensus seemed to be that they were both great against scratches, but that the Brain Cell gave more protection against bumps and drops, and against being crushed by other books in a bag.
I bought a vertical Brain Cell, but haven't bought a backpack for it yet. I'm a big guy, so I was very wary about buying backpacks without trying them on first. In my experience with buying a camping rucksack, it's like Harry Potter wands: you don't choose the backpack, the backpack chooses you.