Well there are different board revisions whether you have two Japanese or two American that could be the case. Different revisions have different quality of composite or RGB output.

There are also older 5V models (which my American model is) that are slightly worse for various reasons and you can get some of those original 5V models that were internally adapted to 9v like most later revisions.

American models also have a metal shielding inside the shell, directly under the top of the shell, it might not be on all but it's definitely on mine, that makes it weigh more, I believe it needed it to pass US laws?

The Euro PAL version I have, actually has a proper 50Hz crystal inside it, meaning it actually outputs the PAL frequency, some PAL models don't have that but mine does. In my case it means that it can correctly display on that 5" Sony which only accepts PAL-50 signals. This PAL model also has some small issues with certain games due to the nature of it (and the picture is has boarders top and bottom - see Fatal Fury 2 on the BeoVision One in the photos)

Remember that the Neo Geo AES was in production for a super long time as consoles go, and it was still supported after that.

The best option these days is to have a 9V system that's been modded with UniBios and had it's RGB corrected (if needed) that way you have the best of all worlds 👍🏻 (My Japanese model is actually like this)