I am a huge fan of "the mule" by camelback. It holds 100oz(2.7L) and has a small pocket for lip balm & wallet, a bigger pocket for food, and a nifty mesh that held my .357 quite nicely. Also if you put cold water in it the insulation keeps it cool for~1 hr in extreme heat. Used it last year in Az on lotsa hikes. They even have a model that holds 2 100oz bags
When I bought the camelback I looked at a bunch of others. I liked the sturdy shoulder straps- they made the load quite comfy. Some of the models really chinze out here. Also I prefer the slit for the nipple end instead of the pushbutton. The slit never leaks but the pushbutton always seems to.
Compared to canteens- well there isn't really a comparison. The "backpack" is barely noticible and completely out of the way(BTW the waist strap is useless). With canteens you have an uncomfortable load around your waist and its cumbersome to get a drink. I never had any problems going through brush.
wish I had thought of it