I’d say you’ve already answered the question, Justin. Something like the CamelBak BlowFish ($90; www.camelbak.com) is designed to carry a hydration bottle and some gear, but its 864 cubic inches of cargo space will fill quickly with just a jacket, lunch, and a few Snickers bars. The alternative is something like the Gregory Scram ($120currently $79 at REI), a 1,600-cubic-inch daypack that also holds a big water bladder for on-the-go drinking.
Really, that’s itthe difference is in the load you want to carry. Because the BlowFish is smaller, of course, it’s slightly better for things like mountain biking or trail running because it’s less apt to swing around. But the Scram will also work well when you’re doing high-motion activities. The bladder part is the same; I mean, a water bladder is a water bladder. Companies go through all sorts of agonies to come up with better bite valves or easier-to-clean bladders, but the basic technology is really prettyBwell, basic. I find it somewhat amazing that gear makers have managed to come up with a gazillion variations on the canteen-on-your-back theme, with prices from $40 to well over $100. It’s a real triumph of marketing over need.