Jay, if youi're doing techno that has some significant changes in it (or just different projects for that matter) you might find that one will work better than the other in certain cirumstances. Some people use both but, from my own experience, it's a bit over my head I think since I haven't found an advantage of using both at the same time. Generally, I use compression more than ducking for anything else. But, you've heard some of the shit I've worked on which means that don't necesarilly mean a damn thing. ;)