What is the comparative value of armor proficiencies?

The intent is that I want to playtest some racial features in a restricted setup, before using them in more complicated homebrew endeavors. Applying medium or heavy armor proficiency to this race seems like the easiest way to playtest medium / heavy armor proficiency as racial feature.

The way I see it, it seems that light and heavy armor have other restrictions than proficiencies, such as the reliance on Strength or Dexterity and, in the case of heavy armor, stealth disadvantage. On the other hand, there are no races that grant heavy armor proficiency and there are quite some (sub)classes that grant access to light and / or medium but not heavy armor.

Is medium or heavy armor proficiency as racial feature more powerful (in general) than light armor, or is it just favoring different kinds of builds?

I will use armor proficiencies on homebrew races in the future but first playtest it on the hobgoblin. I would therefore prefer a more general analysis, but considering the specific example (see below) would of course be appreciated.

Given the hobgoblin race with the following feature:

Martial Training. You are proficient with two martial weapons of your choice and with light armor.

would the race become more or less powerful when granting medium or heavy armor instead of light, or would it just lend itself to different builds?

I intend to use the change with the optional rules from TCoE concerning the customization of ASIs. Therefore, the specific ASIs of the race should not be relevant for the analysis.