Hi...my two cents worth...
Backpacks in Australia & New Zealand are alway smore expensive. Unless you happen to stumble upon some great sale. Even the packs on ebay australia and NZ is more expensive than packs here (most of the time)..
I have to agree that a dry bag of this size (like the picture above) with carrying shoulder straps are not the way to go. Dry bags are made using lamination/ seam technology. They fuse the materials together. They don use sticthing because it punctures the material, hence water can get in. With lamination/ seam products, if you put heavier load, they tend to stress too much and this will lead to tear.
How I know this? Because I have used them before. Some more the one I use is not cheapo brand, it is from Wytex!
So...my suggestion is...if you wanna use big capacity dry bags (40 liters above) and you wanna carry it like a backpack, use it with a skeletol carrier, like the ones below:
http://www.nrsweb.com/shop/product.asp?pfi...33&pdeptid=1989http://www.equinoxltd.com/the-gear/backpac...pamola-pack.cfmAs for the brand of packs, it is to each persons choice. Its almost like a car. Some with more cash would prefer a BMW, while other are happy with a Toyota. At the end of the day, it serves the same purpose, brings you there. But it would be a shame to own a BMW X6 and not be able to use it offroad

thanks again.
i'm not picky about brand as long as it can serve me well and the 'toughness' of the bag