Just one of the many things I've had to adjust to by switching from road running to trail running is the need to be able to carry water. Lots of it. I was always able to plan road running routes around the location of water pumps and drinking fountains - not usually an option out in the woods!
Last year I purchased the CamelBak Day Star, a women's model designed for day hikes. Holds 2 liters of fluid, has a bite valve which is easy to operate and doesn't leak, a fair amount of storage space inside the 1 organizer pocket, 2 compression straps and a waist belt that allow you to customize the fit, and the model I bought wasn't pink (I HATE pink). Total weight of the empty pack is 1 lb 13 oz.
I found the CamelBak to be very comfortable while hiking or walking, but I experienced horrible chafing on the sides of my neck from the shoulder straps while running. No matter how tight I cinched the straps, even to the point of being uncomfortably tight, the only way I could control the chafing was to wear an Under Armour Cold Gear Mock T-Neckunderneath the pack. Fine for cool weather, but doesn't cut it in the heat of summer.
At the recommendation of another runner, I tried the Nathan Intensity Hydration Pak in the women's model. This also holds 2 liters of fluid, has a vertically adjustable sternum strap and compression straps to customize the fit, a zippered pocket for storing gear on the back of the pack as well as 2 smaller pockets on the front shoulder straps where you can reach them without having to take off the pack. Perfect for carrying gels or electrolytes. Total weight of the empty pack is 5.5 oz.
I found the Nathan vest to be light weight, very comfortable even while running, and NO CHAFING! :)) The front pockets on the vest are great too! The bite valve was awkward to use - push in to lock, pull out to unlock - and it leaked if I didn't lock it while running. The compression straps were difficult to adjust while wearing the pack - I had to take the vest off to adjust the straps as the fluid reservoir became depleated. The sternum strap is very easy to adjust. Since I've only used the pack 1 time, some of these minor problems may be that I'm just not used to the Nathan system yet. Even with a looser feel, the vest didn't cause chafing and was comfortable.
CamelBak has the better bite valve - it's easier to operate and doesn't leak.
CamelBak has more storage space.
CamelBak's compression straps are easier to adjust on the fly.
Nathan is lighter weight.
Nathan doesn't cause chafing.
Nathan has pockets on the front of the vest where they are easily accessible.
Nathan has a vertically adjustable sternum strap to help customize the fit.
For day hiking where you want to bring a lunch, guide books, a camera, and a few supplies, the CamelBak is the better option mainly because it has more storage space. Though it is heavier than the Nathan, it is still light weight and very comfortable.
For running, the Nathan is the hands down winner mainly because there is no chafing. The lighter weight is a bonus, and the front pockets make gels or electrolytes easily accessible without having to take off the vest.