To offer a different perspective: I find XML simpler in that it tends to correspond more closely to most arbitrary data models. From ADTs to C structures, there are product types and sum types to encode, along with more or less arbitrary basic types, which have text representation. With XML you get a straightforward way to encode all those, while in JSON there is no single agreed upon method to encode sum types, but you get something like string-indexed arrays (or hash tables) instead, which are normally implemented on top, using simpler types (and are not restricted to strings). And a few arbitrary built-in types, but you have to use strings for others anyway. And no built-in extensibility, so ad hoc hacks are used when it is needed. It would be particularly awkward to use for documents, too.

I preferred JSON initially myself, since it seemed a little less verbose, I did not care about extensibility, did not consider its usage for documents, validation, did not care about using it for different data models, and it just seemed simple (FSVO) and straightforward. But then more of XML made sense. It is not perfect for everything, either, but the decisions behind it seem more justified to me than those behind JSON (though JSON still fits JS, at least).