Patches can tell you what sort of "crap" you have in the oil and Ferrography, as far as I am concerned, will give you a better indication of what type of wear/ cutting you have going on in a component.
We use both DR & Analytical Ferrography along with patches. The biggest difference between DR Ferrography & patches is: DR Ferrography uses magnets to align the ferrous material so you can really focus on it. The majority of non-ferrous material has a tendency to get rinsed off the slide.
We make patches using several different patch sizes i.e .4 micron up to 11 microns depending on what we are looking for.