There are a few key factors to consider when deciding which hydration bladder is right for you. The most important factor is the capacity of the bladder. A 2L bladder will hold about 64 ounces of water, while a 3L bladder can hold 96 ounces of water. 2L hydration bladders are great for shorter adventures or when you don't need to carry a lot of water. They're also lighter and more compact, making them easier to pack. However, they won't last as long as a 3L bladder, so you'll need to refill more often. 3L hydration bladders are ideal for longer days out or when you need to carry more water. They're bigger and heavier than 2L bladders, but they'll hold enough water to keep you going for hours.