The way it worked in UE3:
You would make your diffuse, spec, norm, etc. maps in Substance Designer, export them as a “substance” file, and then import that substance into UDK. UDK would read the substances sort of like “dynamic textures” that had adjustable parameters (which you would define in Designer). These textures would be applied to materials like normal image textures. Only they weren’t normal, they had dynamic parameters, etc. dirtiness, color, or whatever else you wanted.
Of course, you could also export the textures you created in SD as normal images, and use them that way. They just wouldn’t be dynamically adjustable in UDK. (Apparently this is how UE4 works.)
So Substance Designer is basically a more powerful, texture-oriented version of the UE material editor. Plus it comes with awesome mesh-based features like curvature map baking. Just get the trial and mess around with it if you aren’t sure.
Hope this helped 
(somebody correct me if I’m wrong btw, I’ve only played around with it once or twice. :))