You're correct that direct heating is more efficient - heat loss is dependent amongst other things, time - no insulating jacket is 100% thermally non conductive (none that I know off anyway)
As for the 2 you posted I don't know which is more thermally efficient as there are no specs attached. With saying that, all our vans have 2inch insulation board around each tank and over the top but not underneath and at most, the water temp only drops 2 degrees over the working day. Each tank (650ltr)has an immersion heater (3kw) and we preheat each tank to 30celsius each night in winter if it's forecast to freeze overnight. We also recirculate the water through the pumps and hoses back into the tank through the night running the varistreams at 1 so as the hoses etc don't freeze.
Works for us and ensures our guys get to work in the morning.