When I am contemplating a major pack purchase, I like to rent or borrow the model first and give it a good trial - at least an overnight. Teslinhiker is absolutely right when he talks about individual fit. What works for me may not be as good for you, and vice versa. Be aware that many of today's packs have fairly complex and intricate adjustment systems. The larger the pack, the more crucial these systems may be. I would think you would do well with something no larger than 4000 cubic inches, or even less, down to about 3500 cubic inches.

There are many fine brands of packs today. Even mediocre ones today are better than the top brands of a few years ago, although most "tactical" packs are rather unsophisticated, although they are tough. I use a Maxped Vulture II as an archaeological field pack - it stands up to rough work rather well, although it has some fairly laughable features that evidently give it tactical cred.

Don't forget you can modify and adopt any pack to your heart's content. It is easy to add custom pockets and the like.