Thin.
I have both. I make this recommendation not based on weight, but on design. The thin is designed to be carried as a rectangle (albeit a thin rectangle.) You carry it empty or loaded to max, it always carries and hangs from the hand the same (proper) way.
The original, however, is designed to be carried as a triangle. Look at it from the end, when empty ... a triangle! You can kind of see it here:
Carry it empty, and the handle is perfectly situated at the apex of the triangle so the bag hangs down straight. Toss in a few smaller items, and no change. Load it up as much as you'd load up the "slim" and no real change either. But then, as you get closer and closer to loading it up to it's apparent "max capacity", the geometry of the bag changes from triangle to rectangle ... but with the handle at the same spot on the top flap, it moves from the centre of the bag towards the back ... this photo of an Amazon knock-off sort of shows this problem ...
The result being: the bag does not hang down properly from the handle, but at an angle (with the bottom edge of the bag hanging in towards you and digging into your leg as you carry the bag.)
I actually went with the Amazon version, called the Polare -
http://www.amazon.com/Leather-Shoul...&redirect=true&ref_=oh_aui_detailpage_o04_s00
"Doc4" is correct that if you max out the carrying capacity of the Amazon version it is heavy and not as easy to carry by its handle. However, the briefcase/overnight bag is more easily and comfortably carried using the shoulder strap when you are carrying a full load.
Also, the price differential is something to consider. The Amazon Polare is very well-constructed of thick leather and runs $200 whereas you will pay 3.75x that much ($749) for a Saddleback Leather Co. version - the US Waterbag.
All-in-all I am very happy with my Amazon Polare. Speaking of which, I used it recently while attending a conference in Atlanta, GA and was able to pack two full days of clothes (blue jeans, shirts, underclothes, socks), computer tablet with mouse and power cord, a puffy soft leather-bound notebook, and a stack of papers in an accordion folder.
Tim