using two way is very complicated I think because after two weeks from project start we could get 1000 modification sometime. and searching where is the fill atribute and what is the actualy one is very annoying :)

some cases:
1.,
We have got a layered wall with three components (structure, insulation, plaster)
the whole wall is one Object. but the three components have sparate fill sign.
In this case the fill atribute is better for Styles eaches.
2.,
We have got a layered wall with three components (structure, insulation, plaster)
the whole wall is one Group. but the three components have sparate fill sign.
In this case the fill atribute is better for Objects eaches.

It is really dificult it depends on a lot of things mainly the program structure I gues. :S
But my my opinion that using with only one place Object OR Style.