It all depends on what you intend to use the pack for. Is it for a long excursion where you need to carry a large stock of supplies and equipment? Or simply a light pack for a one-day hike? Consider price as well. Packs can get very expensive very quickly, so it's important that you know what you want the pack to do. For example, Swissgear, the makers of Swiss army knives, make a very affordable 34 liter pack for $22. But again, you might need something more heavy duty.