What race is the retard? What sex?
Seriously though, I would kill the retard instead of the chimp. My reasoning:
I've worked with many delightful individuals who were mentally retarded. They all knew they were too, and were quite intuitive about the social stigma attached. They're retarded, but they're not stupid. I'm not saying it's impossible to enjoy life in that position, but you really do become something of a social leper, except in a few cases when you have an especially understanding and supportive community.
The chimp has a lot going for it - if in the wild, it will most likely succeed, as well as being high on it's social order, and living in a meaningful community. It may even sharpen a stick or something.
If it lives in a lab, we can study it's behaviors and learn more about ourselves and lesser primates. Animals in captivity also form strong social bonds with they're handlers, and being socialized by humans increases many animals latent intelligence.
Let's be honest - regardless of our side bias, there are plenty of human beings in the world, and one more won't make or break the species. For many chimps, that is not the case, and one gifted chimp could make the difference between success or failure in the region for the species.
Let the retard have it, let the chimp go.