I found a Stack Overflow thread where people discuss this and actually test it. Further down the page is a 2026 test. The results are it's basically the same, very little practical difference in normal usage. If you're auto saving a file every 30 seconds you will not notice any difference between XML and JSON.

Personally, I like XML more for data files like this. I would not reach for JSON as my first thought for deserealizing a complex object hierarchy. Stuff like, a polygon can have a drawing of type vector which can have n paths which each have exactly 2 points… easy to store and validate in XML. Quite messy and complicated in JSON.

<Drawing type="polygon">
  <Line color="#fed1aa">
    <Point x="6" y="7" />
    <Point x="69" y="420" />
  </Line>
</Drawing>