Let me weigh in on this one! I was a portable guy from 1990 to 2000 when I went to mostly truck mounts. Heat does make a huge difference! For a long time I used Dirt-X-Tractor portables and they worked really well for me. The concept of the machine is essentially it is a big wet vac. It uses a weighted tubular drag wand of their own design fitted with two 11006 jets. An adapter is hooked to the faucet, a solution hose is screwed onto that and it uses the hot water from the faucet. If we were in a place with no hot water, we used a little giant hot water heater. We presprayed hot solution down, then rinsed it out with the drag wand. It was a really effective cleaning system, and that company today is still doing the majority of the carpet cleaning in the town they are in, including all of the high end work. The only thing I didn't like was the dry time was 6-12 hours or sometimes longer if the carpet was really dirty. If I owned rental property and had employees that would be running the machine, I'd buy a Dirt-X-Tractor in a heartbeat. They are indestructible.
I went to using a ninja high pressure 500psi portable w/heat, and that worked really well for me. On a hot, low humidity day I had a one hour dry time, on really humid days it might be as long as 8 hours (In Louisiana it is REALLY humid!) The heater had no trouble at all keeping up with the pump. I run two 11002 jets in my wand. If I put hot water in
the portable, the wand would "pop" while cleaning, and it would definitely burn my hand on the connectors. It was setup with two two stage vacs, heater, and a 500 psi DC motor pump. With this setup, I rarely had circuit problems. Like Larry Cobb mentioned, I like the AC motors better for durability, the only thing is that they draw more amps. Both motors power the same pump head. The dc motors will last about two years. I personally would rather pay the price every two years and enjoy the benifits of lower amp draw and high heat.
As to what machines offer this, the Ninja by Century 400 of course, the Olympus series (One thing I like about the Olympus is it is taller so easier to get up stairs), TMI (Timbucktoo Mfg. Inc.) also makes a good one. I would steer clear of Mytee. I've heard some people are really happy with theirs, but when I run a service center I had a lot of quality control issues we fixed. The same problem with Century 400's Ginsu.
Addressing auto-fil, auto-dump issue: It depends on what you are doing. You run into the problem of too many amps per cord, and it adds weight to the machine. Don't get me wrong, I like those two features, but there is a tradeoff. If you are doing commercial, you need it, or very large residential you may could justify it. Other than that, I wouldn't get it.
Use this as a stepping stone to get the cash flow to buy a kerosene or diesel fired truckmount. Then you'll really be cooking!
Jeff H.