I don’t think it’s that straightforward to answer that. They’re both body fonts. Public Sans is a bit wider (as it isn’t geometric) and roboto seems a bit thicker. Besides these bits which can be worked around, they’re functionally too similar. Maybe you’d prefer to use Public Sans because it’s less condensed which works well for readability of smaller fonts that would be in a body of text. But you can just adjust a number of things to get what you’re looking for here.

A more vague answer I can think of is that it’s preferential and doesn’t matter to most — with designers just being highly particular about preferences, in a way that isn’t really open to objective choice. One font may display slightly better but the other font pairs better with the title font. Or we’ll look for specific issues that I don’t really see in either fonts.