From a visual viewpoint the "Gearsack" brand bag, is made of thicker material, it's inner waterproofing layer also appears to be thicker, hardier and generally more durable. The storm flaps have greater depth and therefore coverage, the zippers are much sturdier and the Piping is a tougher thicker rubber like material.
Inside the bags are very simliar. Capacity is slightly (around 5 litres I think) smaller in the "Gearsack" bag than the "Ventura" bag. They have the same amount of Storage pockets, just arranged a little differently, but the "Gearsack" bag has a neat little Card/Licence/Credit card holder, and a Map/Paper and pen storage area, attached to the inside of the lid, which is really quite handy, and in the waterproof section also.
The area on the 'Ggearsack" bag which mountes to the rack loop also is much thicker in material, and the stiching is more robust looking from appearance. I can pull a "Ventura" bag with my hands and watch the sticthing stretch, whereas I can't with "Gearsack". It can be used with "Gearsack" and "Ventura" racks.
The "Gearsack" bag also came with a carry strap as standard. Whereas, the old one didn't have one, it just had a handle.
Once I have done this next trip to Sydney, I will let you all know how it fared in comparison on a trip!
