I mentioned in a different thread that while there are tons of similarities, there is a core thematic difference. Chris's story was always destined to be a tragedy. Being born with his mental issues and the parents he did, all paths led to prison or a mental institution as he got older. He was always going to find it harder and harder to be unable to interact with the world and do something to get on the government radar.
Ralph on the other hand was destined to be a comedy. Ralph isn't retarded, had an opportunity of a college education, a moderate home, hell, even a loving wife with a potentially very successful career. At every step of the way Ralph takes every gift he's given and shits all over it. Ralph chooses to destroy his life purely to spite other people who have happy lives. This is a man actively and consciously choosing the worst decisions.