The bad news is that the most important thing about ANY pack is how it fits you and until you have put your full load into a pack and carried it around for a bit, there is no sure way to know how well a pack will fit your particular body. If someone pipes up and says they own the pack and love it, this will tell you very little about how you'll like it. If your pack doesn't fit and you end up wearing it as a daily millstone, you will regret it.

This method of loading up your pack in order to find out how it feels on you is also a perfect opportunity to see if you can fit what you need to bring inside it. Remember that you will probably be carrying at least some food and water, so you should simulate this part of your load, too.

The good news is that REI has a very good policy on returns, so you could try both packs with very little risk other than paying for return shipping. If that seems too costly, then you should plan on going to a store that carries a wide array of packs, so you can do your evaluation on a great many of them at once.

Finally, all else being equal, a lighter pack is generally the way to go. Those 5.5. or 6 lb packs may be comfortable, but you'll lug that extra weight every step you take and waste energy you could use for better purposes.

Good luck.