I suspect that if we unpacked what they meant, it would be somewhat different. My hunch would be that Cameron would define them as the opposite of so-called benefit scroungers or work-shy, whereas Miliband would probably see it more as way of talking about the employed working class without sounding like a leftie and scaring the Daily Mail et al more than he already does.
My heretical thought is this: what is virtuous about being a "hard working family" anyway? What's wrong with being a working family, who earn enough to meet their bills and still have time and energy for other things? What about families where no-one can work, due to illness, disability or a lack of available jobs? What about people (hard working or not) who haven't got a family, or at least don't live in a family-type household?
The problem with current rhetoric is that it implies that anyone not fitting the "hard working family" template is somehow less than ideal. Policy is being geared to this heroic group of people rather than the isolated, unemployed or marginalised. We already live in times where benefits sanctions are readily applied to anyone not able to comply with demanding conditions, sometimes in cases where good reasons prevented people from doing what was required. In such a punitive context, when all assessment of budgets and policy seems to be based on "what's in it for me", public opinion could very easily be tipped towards even greater restrictions on the support and help we give to the vulnerable of our society from the taxes of those the politicians charm with their hard-working rhetoric.