In reply to Loughan:
They are all different and will all fit you slightly differently.
The feel of the sack on my back is important, and something I pay a lot of attention to. So I suggest you try them on before deciding.
I have an old 40l Crux rucksack in the kevlar fabric, and it has withstood all manner of abuse.
I also have the old Crux 40l in the lighter weight fabric, and it has not withstood abuse nearly as well.
It is also worth checking out the Tesco sacks - they make some good kit at very attractive prices. Worth looking at Argos too, and seeing what is in the various remainder shops. I got my current expedition sack from Leckies in Falkirk - a GoLite 80l job I haven't seen elsewhere in the UK. Light and comfy, but not very abrasion resistant.