MP4 vs MKV quality

[HMR]

Hello everyone,
My question's in the subject - I try to cut off some parts of my videos (in "copy" mode), and I choose between MP4 or MKV as output formats.
I know MKV is incompatible sometimes but I don't care, all devices and players I use support MKV ok
What I would like to know is the output quality. Just visual quality of the video, not additional subtitles etc.
I guess in copy mode quality should be same, no matter what I choose - MP4 or MKV, since they're just containers.
But there are many articles on the net that say MKV has better quality than MP4! That confuses me.
Which is correct? If I save part of the video, in copy mode, using MP4 or MKV - they will look same? Or MKV will look better.
Sorry I think it's lame question again. But net search gave me so different results...
Thanks!

[eumagga0x2a]

I guess in copy mode quality should be same, no matter what I choose - MP4 or MKV, since they're just containers.

Exactly that.

But there are many articles on the net that say MKV has better quality than MP4!

MKV as container has features MP4 is missing and vice versa. These features have nothing to do with quality of video and audio stored in the container.

[HMR]

Ok, understood!
Some net articles clearly say about different container features, but some don't and just mention something like "MKV quality gonna be better/MP4 quality is bit lower" etc. So I thought it might be about output files' video quality after all.
Thanks for reply!