I reckon the people of the first are just as strong (and nutritious, I guess ) as people from the Source. Primarily because soul density just doesn't seem to be all that relevant outside of very niche circumstances, like say for example, if you're trying to munch down on an entire continents worth of light aether.

Basically everyone interacts with the world using a mix of pure physical strength, aether drawn from their body, and aether drawn from the surrounding environment. Outside of a couple main characters I don't recall an instance of someone using their soul as fuel for their abilities.