Brain Cell or Monolith
I am trying to decide between the brain cell and the monolith for my 15" Powerbook G4, aluminum.
Currently I have a very old Eddie Bauer soft-sided bag that I use; I wrap my powerbook in the foam sleeve it came with and carry it in there. Financially, I will have to wait until later to buy an entirely new overall bag/full-sized carrying case, and I am not sure if I will want to eventually get/use a backpack, messenger-type of bag, or a traditional briefcase, so I currently need something that would be most adaptable to all of those possiblities.
Personally I would say that a 15'' is big enough to make the difference between the M and B significant. Looking at the measures at the rightsized Monolith I'm pretty sure it will not fit in most messengerbags or suitcases. So in other words whatever you buy now sort of "locks" you into a certain type of bag. I.e. either a backpack or a landscape type bag. Keeping that in mind I would say the small favor goes to the Braincell since due to it's exterior pocket it's by far the best bag to use all by itself. With the Monolith you easily end up without a place to keep the essential accessories like the powerbrick, or a mouse. But of course I might be a bit biased since I already own a Braincell :)
For most Tom Bihn bags, the Brain Cell is your best option. The only bag in our current line that it does not fit is the Smart Alec.
For other brands of bags it is just a matter of whether you will go with a messenger bag/briefcase or a backpack. To get the full protection of either of these bags you want to keep them in the orientation that they were made to be in: horizontal for the Brain Cell and vertical for the Monolith. This is because the bags have built-in suspension and you'll lose that if you turn them on their respective sides.
Backpack = Monolith (except for our Brain Bag, which is wide enough to hold the Brain Cell horizontally).
Messenger bag/Briefcase = Brain Cell.
The exterior dimension of both of these bags are:
Size 4 Monolith: 10 x 15.25 x 2.25"
Size 4 Brain Cell: 13.5 x 10 x 1.25"
Hope this helps. Let me know if not.