Then L: grew up with no family, no integration, no urban landscape, no connection to the outside world except through a detached and highly theoretical understanding of it. Again, theoretical in the sense that he has strong ideas about justice like Light, but also theoretical in that unlike Light he never has a chance to practice and adapt to social norms: he remains detached and aloof from society as part of his job, but also part of his personal predilection. Unlike Light who actually functions better from directly within the society he's observing, L functions better outside of it. Light represents pure integration, L represents pure disconnect; Light represents the urban landscape, L the rural/suburban; Light represents societal progression, L represents traditionalism.
Yet for all that L is the alienated one and Light the fully mainstreamed one, Light is the one whose ideas ultimately disconnect him the most and who winds up presenting the most regressive ideology as a purely idealistic, futuristic one. L is the one who manages to hang on to the truest picture of humanity, along with his hope and his idealism - his disconnect from society actually aids him and maybe even allows him to stay rooted to its traditional values. Light's version of humanity is one that is warped and cynical - his idealism, too, is a warped and cynical vision of society's potential: his ideals aren't "real" ideals any more than his golden boy persona is a "real" persona. His fully integrated view of the social system has left him with no expect for it and a perfect knowledge of how to manipulate it.