"Cost is not a factor in many cases."
Up to $2400 versus up to about $500, that's a pretty big factor to consider...
Filtration on current Sanitaires, well, they use the Filtrete ST bags, which are huge, bigger than Kirby's own version, so more surface area means more airflow just from that alone, plus the bags are better at keeping the dirt in the bag compared to the plain and Filteraire F&G type bags. Yes, a Sanitaire is a commercial vac, but you have to remember, they turned a Commercial vac into a Household vac with the Eureka F&G upright series, a lot of which still outclean the competition today with tiny little 3.5 amp motors, and they're still selling the same basic design today, brand new with a range of different power & style of motors and a range of brushrolls...
Kirby on the other hand, their machines have gotten weaker in terms of cleaning, they're heavier than the likes of the Dual Sanitronic80 (which for me is about the same weight as my Sanitaires), and the fact you cannot buy them brand new at a retail outlet, just from someone cold-calling at your door and demanding a rather large amount for something that loses value so fast (I recall the Sentria sold at an auction house in Burnley, £80, compare that to the new price!!!). And you only have 1 model available, 1 motor to choose from, and many functions it's not really that great at...
If you want cost effective, simplicity, ease of maintenance, and performance like no other, go for a Sanitaire...
If you want shiny, heavy, fiddly to use, and expensive, go for a Kirby...
And something you might want to read through (thoroughly) on comparing a Kirby against a Sanitaire:
http://www.vacuumland.org/cgi-bin/TD/TD-VIEWTHREAD.cgi?6053