Here is my opinion
The fact that ASON does is a superset of JSON is pretty cool of course.
ASON
- ASON is a JSON superset, avoid to have several parsers in IDEs
- ASON does not account for whitespace (good)
- optional colons and commas, I dislike them
- I don't understand not want ASON nameless fields, even more after the example with "equivalent" literals
- for some reason I don't really like unquoted string for values, sounds like easy to forget a word and offset all the parsing silently
SDL
- SDL has a readily available parser package sdlang-d, in a stand-alone library
- SDL account for whitespace (EOL) if I'm not mistaken, ASON does not
- SDL has a "date" data-type which seems useless for DUB purpose
So I was sceptic at first but after writing down pluses and minuses I would favour ASON indeed, and more so if the format becomes stricter.