It really depends on what Im doing and how expandable I want it to be. If I am making some small test environments, where layout and such isnt so important then I use modular. But I will generally always use the unique approach, where I model out the walls, floors, ceilings, ect as separate unique meshes, this gives me much more control of the look I want, as well as it reduces the issues with seams and lightmap errors that come with the modular approach.