The advantage to a Demon is very high waterflow--and recovery--I put a valve on mine to be able to adjust it depending on facilty pressure. I keep a bottle of defoamer (dry) on my machine and put it down on the carpet in the spots that foam up.. easy to figure out.
Auto feed and auto dump are standard and very simple to hookup. I remove the shower head or hook to the laundry hot water if the washer isn't there. Whereas it is a lot more of a hassle using a porty with auto feed and auto dump (my opinion).
I have found if you use a little agitation they clean fine with cold water and good chems---I have cleaned apts with the water heater off.
It is obvious Mikey has used one... Backhoe lawn mower says it all. If you do not have a
glide on it you cannot move it forward without lifting the vacuum off the carpet. Always moving backwards drives me more nuts. Mine is the classic model, they make a model that you can reverse and move in the other direction, sorta.
Steps are a problem. I have never had the extension to do steps and upholstery.
I believe recovery is higher than most porties with the demon---tiny waste tank and the vacuum is right there.
I have a little $29 shop vac to suck all the junk out of my waste tank... works great.
One 15 amp outlet is all you will ever need. With a Porty you gotta have two circuits, if you want heat you need three.
Minivan or even a basic station wagon will easily haul demon, hoses and something to agitate with.
I met a guy that uses his Demons to do apartments. He has been doing it for several years. TM, Porty, Demon---he claims the demon is much simpler and faster.---his opinion.
By the way, US products made a knock off.
I am just a hobbiest, I only clean my rental apartments.