A speed bag is small and mounted at the top only. It's primarily used to improve hand speed and hand/eye coordination. Some double-end striking bags are a similar size, but many are larger. Double-end striking bags are designed for a much wider variety of punching and kicking practice. Somewhat confusingly, you can get double-end speed bags. These are a hybrid of sorts. Before deciding what you want, think about the type of exercise you're looking for, and choose a shape and weight that suits your goals.