The Webasto and Espar heaters are fine, if expensive, products. Their use on school buses and like applications speaks for itself.
Be sure to size the heater carefully, you don't want one that's oversized on heater output, as it will cycle on and off constantly. Better to have a smaller one that can stay on most of the time. The cycling will drive you crazy and wear out the heater faster.
As far as the differences between the Webasto/Espar heaters and the Wallas, I'm going to hazard a guess w/o spending the morning fully researching the subject.
The Wallas is a "drip pot" design, meaning diesel fuel is dripped into a hot combustion pot which vaporizes it. (Heating up this pot to vaporize the fuel is what initially takes a lot of amps to start the heater.)
The Webasto/Espar designs, by contrast, inject and vaporize diesel fuel through w hot grid into a combustion chamber, much like the space heaters designed for shop use that look like tank-type vacuum cleaners from 1952, except that they have externally vented exhausts, a heat exchanger/circulation-blower, and electronic controls designed by the NASA and Wallas boys.
Webasto heater
Me, I'm still trying to figure out why we just don't put in a wick powered kerosene lamp/heater with a long metal chimney to let out the heat on the way to the cabin top and some 12-v computer fans to move the air around. But I guess that wouldn't be doing my part to help drive up the discrepancy in the foreign trade balance of payments department.
So I'll just have to stick with my Force 10 Cozy Cabin Propane heater for the time being.
Joe. :teeth :thup