In reply to PixieNinja: I would say get a backpacking bag rather than a climbing bag as you are using it backpacking you want the hipbelt and shoulders to work well, and climbing packs tend to reduce in this area for weight
Go to shops try lots on, with weight in, and ideally not those weight bags they use, actually take all your stuff you plan to take, make sure it fits in at the same time, a good shop assistant will not laugh at you for doing this and be happy to spend time making sure adjustable back systems fit you
go for the most comfortable one, in the end they are all just a hole to put stuff in and most of the features are the same nowadays