In reply to Removed Userplodding painter:
If you can't get everything in a 40 litre sack then you may be taking too much gear.
a) Like the others go to a shop, try a couple on. But don't forget just about every sack I've seen can be adjusted to fit.
b) Don't fall for fancy names like "Alpine warrior", "Force 12" and so on.
c) Buy a well known make.
d) Don't get obsessed with buying the lightest one. A few ounces here or there isn't going to make any noticeable difference when you've filled it with kit.
e) The most expensive ones aren't necessarily the best ones.
f) 'climbers' sacks generally have 'simpler' outsides but if you're scrambling I wouldn't bother whether it was a climbers sack or not. Either way there doesn't appear to be much different these days.