Some of the features to consider when purchasing a leaf vacuum are the kind of power source it has, the type of impeller it has and how it switches from blow to vacuum. There are other features that affect how comfortable they are to use as well.
Electricity and gas are the most common power sources for leaf vacuums. Gas-powered ones tend to be more powerful, but they are also heavier and noisier. They tend to be better suited to larger gardens. Electric ones are quieter, easier to handle and usually cheaper than gas models, although they need to be either plugged in or charged regularly.
Leaf blowers with metal impellers tend to be longer lasting and more durable than models with nylon impellers. The metal is less likely to bend or break when struck by large objects.
The type of handle that the leaf vacuum has makes a large difference in how comfortable it is to use, so it is important to test a leaf blower out before purchase whenever possible. Some vacuums have added portability features, such as wheels at the end of the nozzle that allow you to run it along the ground rather than holding the weight up.