i would say by design Ashbys inline would be the best , pretty sure the Magma , the powerflite port a heat , and the Ashbys steammate are the same heater core ... the one pictured below , dual heater core . The difference is Ashbys wrap a copper coil around the hot metal underneath the insulation , thus increasing the efficiency by increasing the length of heated metal the water passes through , or the time it spends inside the hot metal .
Others like the prochem heat and run appears to be thin single core , and the mytee hot turbo uses four single core units joined , which should give good heater length although they appear shorter lower power units .
So i would pick the Ashbys on that basis . If its fed by water heated by Ashbys intank immersion then u are increasing efficiency even more , so i would suggest either for convenience get an Enforcer with both 3kw heater built in . Or get an Enforcer with just an immersion ... and get the external steammate ... then u can use both at the same time ( just run a cord to another house circuit ) and u could also bring the steammate to the last 25ft close to the wand .
Of course there are other good options from alltec and others but it think Ashbys have the best heating systems by design .
As for the cold water question .... i believe that came about when some ( for mainly sales reasons ) suggested you ' dont need hot water ' but its kind of a trick statement as it implies its somehow a disadvantage or an inconvience to use heat , and that cold is somehow preferable . Where as , heat , will simply , as most know increase your efficiency another notch .
To suggest you ' dont need heat ' would be similar to suggesting you can do without agitation , or you can do without soil emulsifying chemicals , or you can even do without a machine it self and still clean the carpet with a brush and towel .
But the most efficient methods are preferable to most .
