I would second Eurekaprince's assessment of the pros of the uprights and canisters and would also add that a canister provides more efficient versatility--especially when compared to the traditional uprights that do not have the detachable hose. For example, it is quite easy for me to vacuum between my great room (w/wall to wall carpeting), dining, and kitchen areas (wooden floors/rugs) by switching back and forth between the power nozzle and floor/rug nozzle. The narrow space cleaner and dusting brush/upholstery tool are also conveniently usable for the furniture in those rooms. I was even able to readily take care of a cobweb in the corner of the vaulted ceiling (between 9 and 10 feet) by using some extra wands.
Having a "dynamic duo" may work well for some, however keeping carpets clean is also affected by frequency of vacuuming, the timeliness of cleaning up spills or messes, the use of quality door mats and carpet runners, and how often--if at all--footwear is worn (barefooters can wipe their unclean feet with towelettes) which increase or diminish the need for deep cleaning. The quality of the cleaning (a quick once over versus a thorough twice over) is another consideration. Correspondingly, one needs to consider the totality of the floor care treatment. Shampooing and or steaming/rinsing/extracting periodically contribute to the overall condition of rugs and carpets.
Of course, variables are also an important part of the equation: rural or urban (types of contaminants), geographic location (routinely dry/windy versus humid/calm), adults and children (likelihood of messes), or just adults, type of carpet nap (loops or threads) , household or commercial (volume and type of foot traffic), et cetera.
So, I would say the issue is a personal preference--not which one is more effective than the other. What it most crucial is the person(s) who run the machines and use the liquid and dry chemicals; followed by properly-maintained equipment and good quality products.
