This year I hiked about 500 miles of the A.T. I wanted to try on my pack with weight in in before buying, so I went to a retail store where I found two main contenders: Osprey Ariel and Gregory Deva.
I loaded each pack, and wore both before deciding on the Osprey. Why did I like it? It looked nicer, it was cheaper, it was lighter, and it held more. I also found the hip belt on the osprey pack to be more comfortable, though I was a bit worried about the chest strap, which didn't feel great against my collar bones. In the end, I figured hips were more important than collarbones.
After 75 miles I returned the Ariel 65 pack for the following reasons:
1. Front mesh panel ripped right out
2. Major bruising of collar bones: weeks after my 75 mile excursion they were very tender
3. Some part of the pack dug into my lower back, causing a scar that I still have.
I upgraded to the Gregory Deva. I liked the smaller size (I still was carrying a pack larger than many thru hikers), the abundance of pockets (Ariel has 1 main pocket and 1 top pocket), and the PADDING.
Deva weighs a pound more, but it is a pound I was happy to haul for 400 miles. Where the Osprey pack dug into my back, Gregory literally put a pillow. The "load lift" straps made the load much more adjustable. Over the course of the day, I would often wear it in two positions, which brought relief for collar bones and back muscles. With the Deva pack, access to my water bottle became an option due to the clever hidden nalgene sized holder. Front access, alone with 2 side pockets and a front pocket made use of the pack more convenient. I no longer had to take everything out to get my sandals or pack cover.
The hip belt was not as good as my osprey hip belt had been, and sometimes it was painful. Some days I would have to readjust a few times to make it feel ok.
I'm 5'6'', apparently with a short torso, as I needed size small in both packs. Gregory's hipbelts are sized according to the size of the pack. The size small waist belt was good for me, but by the end of the hike, it was pretty close to being pulled all the way together. I usually wear size six, not particularly thin for a hiker...skinny girls, especially with longer torsos, should make sure that the belt with get small enough for them.