What type of flooring do you have? Most canister vacuums work great for bare floors, while uprights are generally better performers on carpeting. But if you're set on getting a canister vacuum to clean your wall-to-wall carpeting or a lot of area rugs, choose a model that either has a powered on/off brushroll or one that has a dedicated powered attachment for carpet.
Is pet hair an issue? Look for canister vacuums that offer special attachments designed to ease pet-hair cleanup, such as a pet-hair brush or an attachment with rubber fingers that helps grab pet hair from soft surfaces. Crevice tools are useful for reaching corners and crevices where hair tends to build up, and standard upholstery brushes can help remove pet hair from furniture.
Does anyone in your home suffer from allergies? A canister vacuum with HEPA allergen filtration is best for allergy sufferers. HEPA filtration traps particles down to 0.3 microns in size, keeping them from exiting via the vacuum's exhaust. That will leave behind less dust, debris and pet dander that can aggravate allergy symptoms.
What's the layout of your home? Maneuverability is less of a concern if you have an open floor plan, so you could safely purchase a heavier or less maneuverable vacuum without a problem. But if you have a lot of tight spaces and hallways, maneuverability is key. Alternatively, if you need to haul the vacuum up the stairs, look for one that's lighter and well-balanced.
Do you want bagged or bagless? Choosing between a bagged and bagless canister vacuum is really about personal preference. Bagged models have the ongoing cost of replacement bags, while bagless vacuums tend to have smaller dirt capacities and must be emptied more frequently, a process that can be messy.
Most canister vacuums have washable filters, so the only cost of ownership to consider is the cost of replacement bags if you choose a bagged model. Most replacement bags cost between $1 and $2 each. There are also generic vacuum cleaner bags available for a lower cost, but some canister vacuums depend on the collection bags for filtration, so choose wisely. Because some bags fit different models, it's also possible to add an additional layer of filtration to a less expensive canister vac by opting for odor-reducing bags or bags with HEPA media that will work with your model.