i have a client who swears by her big vac, but then she also gave me her copy of the clean team book to read, so she's prob got it in her head that it's the way to go. i've been cleaning for a living for 14 yrs now & have pretty much settled on the kenmore canisters. the models i have now are intuition red & blue; they each have some diff features, but some are common to both: long cord (i do use a heavy-duty extension for lg houses) & hose; easy to change from powerhead to floor tool; mini-powerhead for carpeted stairs; wedges on bottom to hold unit on stairs; floor tool swivels to facilitate getting into odd spots; telescoping wand w/ light (both very useful); good suction (some others have even better suction - e.g., miele - but they're way more expensive & quite inconvenient to use)
red vac pros - mini-powerhead fits on end of either wand or hose;
blue vac pros - floor tool is integrated into powerhead, releases w/ a flick of the foot (this does, however, make the powerhead rather heavy); floor tool has a single row of brushes on either side of the suction hole that angle toward it, so u don't just push the dirt ahead of u when u move it forward.
blue vac cons - electronic controls on the handle make it easy to accidentally switch suction settings or shut power off.
red lists at $480, but it's always on sale for $350; blue is normally $600, but it's often on sale for $500. this may seem expensive, but they don't disallow the 1-yr warranty for commercial use, which most companies do.
winding down here finally...i replace a vac every yr (i put a good, hard 10 yrs of use on them in 1 yr), usu switching the old to home use, & giving away my old home one, but u can also get the 3-yr warranty, which covers everything; i just feel that would be unfair of me, since i'm so hard on them.