I am the language maintainer for the Mexican Spanish language.
The Mexican Spanish language does not need a parent language, and it has been so for a long time
The current Moodle 3.1 English language pack (3 MB) has 33313 strings (core + all plugins).
Currently the es_mx language pack (2.7 MB) has 551 English strings missing translation in AMOS
While the es language pack (1.3 MB) has 8201 English strings missing translation
So, the Moodle 3.1 Mexican Spanish language pack is more than twice as large/complete/detailed/thorough (2.7 versus 1.3 MB) as the international Spanish language pack.
As for the CORE language strings, the es_mx has 100% core strings translated for Moodle 3.1 while the es language pack has 91% core strings translated.
Therefore, I think that the logical choice would be that the es language pack used the es_mx as its parent language, so that the Spain and SouthAmerican users would have a Mexican Spanish translation instead of defaulting to the English language strings. But that is my personal opinion and maybe some Spanish language purists might object to using Mexican local expressions in Spain or South America, just as some Mexican University Moodle admins in the past objected to using Spain local expressions in México (and decided instead to make and expand the es_mx language pack).
If you are in Argentina, and you use a decimal comma, I think you would be better served by using the es language pack and configuring your local lagconfig file for the es language pack to use es_mx as the parent language for es. That way you would have the language that you are used to, plus some Mexican-Spanish translations (instead of 'English defaults') that are not yet available in your usual language.
Of course, you can easily install both Spanish language packs and see for yourself. You are more likely to see many differences in the new Moodle 3.1 features. I have been writing and updating most of the Spanish documentation for Moodle 2.6 and newer, and there are many features that have not yet been translated into the international spanish language pack.
The main point in choosing either international Spanish or Mexican Spanish is what character you use for decimal separator, as it affects many calculations (including grades) in Moodle. Lo demás es lo de menos.
Saludos desde la Ciudad de México, tocayo