I'll assume you're asking about the most common Aqua Hot 450D.
Aqua Hot 450D:
16 gallons of boiler fluid
1 electric element
High pressure burner (I've heard it's loader, I can't say)
All in one housing
8 gallons of boiler fluid
2 electric elements
Low pressure burner (it's pretty quiet, but I don't know how the Aqua compares)
Burner and electric elements are in the main tank, distribution panel houses all pumps
Physically smaller foot print
Other wise they do the same thing. Heat more evenly, endless hot water.
We hardly ever use our burner, the two electric elements keep up just fine 90% of the time. Even down to about 20 degrees, but you have to run the burner for showers at that temp.
Where we are now, we're only using 1 element and shower just fine. I haven't run out of hot water yet and I take hot 15-20 minute showers. Not always the case.
2013 DSDP 4347, all electric, 450 H.P. ISL
Air & hydraulic leveling
14 Jeep Grand Cherokee Overland toad