Which Is The Best Backpack For A 3-5 Day Trip In The Backwoods?

One that fits comfortably on your back.

Seriously.  There isn't a more important criteria.  If you don't feel comfy wearing it for days on end, then it will be rubbish for you, regardless of how much anybody else likes that particular model.

And regardless of how adjustable different manufacturers say their packs are, they are each made a bit differently and the pack that fits one person best won't fit everyone best.

Howstuff works reviews.

So I'm afraid you still have to go to the stores yourself and try different ones on.  The store should have 5kg and 10 kg weights for you to try the bags with.  Besides comfort, the things to look out for are:

Adjustabability.

Length of the pack compared to the length of your back.  A long pack on a short back is no good because the weight will end up sitting on your bum and not properly distributed.

Guarantee.  The longer the pack is guaranteed for, the better it's probably made.

Shop recommendation: It's in their interest to recommend something that you'll like and make you want to shop with them again.

Read here about figuring out how big a pack you need if working in litre capacity.

Rule of thumb is that 1/3 the weight of the pack should be on your shoulders, the rest on the hips.  But not everyone is truly comfortable that way.  Personally I like 40%-45% the weight on my shoulders (I have lower back weakness already). So don't think the rules of thumb are necessarily right for everyone.