de fcpx a premier pro

While there are differences between the two, picture quality can be good or bad in Final Cut Pro or Premiere Pro for the same reasons.

Something to double-check in Final Cut Pro when using the YouTube & Facebook preset is that the Resolution pop-up menu is set to a good match for your Project (Timeline).  For example, if your Project is 1920-by-1080 set the pop-up menu to 1920-by-1080 or 1280-by-720 but not to 640-by-360 or 3840-by-2160.  Also, be sure to change the Compression pop-up menu from "Faster encode" to "Better quality".

A key functional difference between Final Cut Pro and Premiere Pro is that optimized media in Final Cut Pro is just ProRes (a very good choice) while in Premiere Pro is ProRes, DNx, Cineform, or any of the supported formats for smart rendering (all good choices, but spans a wide range of workflows and platforms).

You put exactly the same output settings and show us the final video, to see a little what it is about.

That's an interesting find.

I work in both and don't really see any differences.

Of course it will depend on your choice of codecs and settings in AME or Apple Compressor.

Would you say the file sizes are similar or ar her FCPX smaller?