The most important consideration is how the vacuum is going to be used and choosing the model with the best tools and features to match. Most of the basic features of Shark vacuums are the same, but some models have better tools for reaching under furniture or vacuuming carpet.
As of 2015, all Shark vacuums feature technology designed to operate quietly and prevent loss of suction, as well as having swivel steering and brush control. However, individual models have different benefits. For example, the Shark Rotator Professional Lift-Away model can come apart to provide lightweight cleaning on stairs or upper levels, while the Powered Lift-Away is designed to fit under furniture and deep-clean carpets. Shark also makes combination vacuum and steam cleaning tools that can be a good choice for people whose carpets get stained and soiled.
Vacuums also come with different attachments and hose lengths. However, additional attachments can be purchased for Shark vacuums, so this should not be a deciding factor. Shark also makes a cordless vacuum, which may be a good choice for people living in small apartments or as a backup to another vacuum.
Price is also something to consider. Vacuums become more expensive as additional features are added. Buyers on a budget should decide what they absolutely need in a vacuum, then decide which features are negotiable and find a model that is appropriately priced.