A straight suction canister is good for bare floors and above the floor. A canister with a power nozzle allows you to also clean carpet. Some power nozzles will height adjust for the new friezes and shags, some won't. The same is true of uprights: some will height adjust for friezes and shags, some won't. The new extra soft carpets need the ability to reduce suction (one company drilled a bunch of holes in their nozzle to bleed suction). And some uprights have attachments that work well for above the floor cleaning, but some have none, or they don't work very well. Most uprights cannot turn off the brush separately from the motor, but a few can. This feature allows you to vacuum a bare floor without scattering dirt all over. Also, if you have rugs with fringes, or fragile antique rugs, you might want to be able to turn the brush off. Most canisters with power nozzles allow you to turn off the brush. On the uprights that can turn the brush off, some do it by a lever on the nozzle, others have 2 motors with a switch right on the handle which allows you to turn the brush motor on & off with a flip of your finger, rather than bending over, -- much more convenient when doing fringes.
Some uprights, like the Orecks and SupraLites, are very light. This makes them very easy to carry from one level to another or to transport back and forth in a cleaning business. Other machines, like newer Kirbys and certain Hoovers, are truly self-propelled, where they have a transmission that runs the vacuum in both forward and reverse as you push and pull on the handle. These can be operated with one finger, but are heavy to carry up and down stairs.