Comfort is the most important factor.
And when you're carting around a backpack the size of a block of flats in 40 degree heat, you're likely to regret it. Pick the brains of the sales assistant - they will undoubtedly have travelled themselves. And it is a good idea to put a few heavy things in a pack when you try it on - it will feel a lot different when it has more than just paper stuffing in it. Once you get it home, fill it with your stuff and practise walking in it. Prices vary enormously and while you generally get what you pay for, price does not always mean quality.