I just owned both lenses and in my opinion you have to search the difference in the bokeh:
Pictues taken years ago with my K10D both at f2.8, but when you take a look at the background and there specialy too the chair, you will see it.
The DA40 is the more modern, harder lens.
The FA is clearly the more smooth rendering lens. It has a buttery bokeh.
After a long time with both FA and DA lenses I think the FA lenses render more creamy. I dont know if the words mean the same in English, but in German we say that the FA lenses are the lenses with the warmer way (on the way to the yellow) to show pics. DAs are colder (goes to the blue) with more contrast. Its a thing of like or not like and no quality issue. Like Leica and Zeiss lenses in the rangefinder world. I love the clear contrasty way of the fantastic DA35 for all day and macro shooting, but prefer the beautiful rendering way of the FA77 for portrait (sold my DA*55 for that reason).
We are very lucky on the Pentax planet that we can have both worlds in one bag.
Rainer