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Further to what @chiroshi said, fit is important, features are important but also making sure it carries everything you're taking comfortably. It's not just about volume.
E.g. One of my packs is 42L and weighs about 300g. It has no frame, no hipbelt and is just a cavernous hole. That probably wouldn't be comfortable for what most people are carrying, but is for what I have.
So: what are you carrying? Are you backpacking? Tent? Sleeping bag? Guessing not if you fit stuff into a 33L pack?
Please can I be a bit lazy because of short notice and ask what hiking back pack people recommend for a five day walk? Last year I crammed everything into an Osprey Talon 33l and it wasn't happy so I'd like something in the 60L range with a budget around £200. Thanks lufgus.