There is a variance in the displays because of the different render engine, also before starting work on anything take off post processing such as HDR and real camera settings because these are for the final render (taking a pic to display).
Once you have disabled the post processing and you have applied smart materials etc on top of your basic defuse map and scratches, dust, etc and then export (I use the UE4 Profile) and export to Jpg, Taga etc then when your setting it up in Skyline then the results are very close to what you had in the plugin (quixel Suite), since quixel is a set of plugins inside Photoshop when you export your maps they are as you would expect exporting as normal from Photoshop, meaning what you see is what you get in the saved maps.
The exported map's can include (dependent on profile or you can write your own), Albido, normal (multi normal as well), AO, Spec, roughness, metalness and more, meaning that your maps are fully supported by Skyline and other engines.
Mesh (model)You can import your model to paint on, deffuse, dirt, damage, etc directly. The painting process is none destructive until you accept masks and then if you export it creates separate maps to the base ones you may have in the first place.
BatchingYou can also batch many models in this program.
My OpinionIn short, to your question
there is a little varience in the displays in engine and Quixel suite but when you have your light setup correctly and the correct maps in the engine the results are very good and very similar, The key is to use roughness maps these make the output look great in engine.
If your designing and creating models then this is a great tool to have in your toolbox, it speeds up your workflow to no end.
If your importing a mesh into 3DO (you dont have to) as long as you have UV maps for your mesh it will enable you to paint directly to your model.
I would get the Suite and not the separate programs, the reason for this is that NDO can produce some amazing normal maps, it also has profiles to create photo realistic normal maps as well.
I hope I have answered any questions that you may have
or might have in the future