And in your case, I'm not entirely certain you're heading in the right direction. If I read your question correctly, you're after a pack to use for overnight hiking trips and biking trips. Right? Well, for one thing, I wouldn't agonize over whether or not you have a "hydration" pack. Really, any pack can be a hydration packjust slip a water bladder into it, thread the tube out, and you're all set.
With that in mind, though, it's true that the Ultimate Direction SpeeDemon ($120; www.ultimatedirection.com) is a pretty good crossover pack. Its 2,070-cubic-inch capacity (not including bladder) will force you to pack VERY frugally. But these days, with all the good-quality lightweight gear out there, it's possible to work with this minimalist pack size. And, its low profile will ensure that a light load while mountain biking will not throw you off balance. The same holds true for the CamelBak Peak Bagger ($100), which maybe isn't quite as nicely designed as the SpeeDemon, mainly due to its lesser suspension.
Really, though, any good-quality compact rucksack will work pretty well. Marmot's Talus ($139; www.marmot.com), for instance, will let you pack a bit more for an overnight trip, while staying sleek enough for biking. Just stick a bladder inside and you have your hydration pack. Same goes for Gregory's Shakra ($129; www.gregorypacks.com), or the Arc'Teryx Bora 30 ($95; www.arcteryx.com)both are great daypacks that have about 2,000 cubic inches of capacity and could be retroffited with a bladder.